Pixel circuit, solid-state image pickup device, and camera

ABSTRACT

A pixel circuit has first, second, and third field effect transistors integrated and connected in series from a photoelectric conversion element to a side of an amplifier circuit. The first and second field effect transistors have gate electrodes to be simultaneously collectively driven. A threshold voltage of the first field effect transistor is set to be higher than that of the second field effect transistor. As the gate electrodes are driven step by step, electrons generated by the photoelectric conversion element and transferred via the first field effect transistor are accumulated in a channel region of the second field effect transistor. The electrons accumulated in the channel region are transferred to an input of the amplifier circuit via the third field effect transistor.

TECHNICAL FIELD

The present invention relates to a pixel circuit represented by a CMOSimage sensor, a solid-state image pickup device, and a camera system.

BACKGROUND ART

Recently, CMOS imagers for a digital still camera, a camcorder, asecurity camera, and the like have been widely used and markets havealso extended.

The CMOS imager converts light incident on each pixel into electrons bya photodiode, which is a photoelectric conversion element, accumulatesthe electrons for a fixed period of time, digitalizes a signalreflecting an accumulated electric charge amount thereof, and outputsthe digitalized signal to the outside.

FIG. 1 is a diagram showing an example of a pixel circuit including fourtransistors in one unit pixel.

A pixel circuit PX1 of one unit has a photo diode 1, a transfertransistor 2, a reset transistor 3, an amplifier transistor 4, a rowselection transistor 5, an accumulation node 6, and floating diffusion(FD, a floating diffusion layer) 7.

A gate electrode of the transfer transistor 2 is connected to a transferline 8, and a gate electrode of the reset transistor 3 is connected to areset line 9. A gate electrode of the amplifier transistor 4 isconnected to the FD 7, and a gate electrode of the row selectiontransistor 5 is connected to a row selection line 10. A source of therow selection transistor 5 is connected to a vertical signal line 11.

A constant current circuit 12 and a sensing circuit 13 are connected tothe vertical signal line 11.

In the pixel circuit PX1, light incident on a silicon substrate of apixel generates pairs of electrons and holes, and the electrons thereofare focused and accumulated in the node 6 by the photodiode 1. Theelectrons are ultimately read as a signal directed to the verticalsignal line 11.

Hereinafter, specific operations of electric charge accumulation andread will be described with reference to FIG. 2.

FIGS. 2(A) to 2(D) are diagrams showing a timing chart of the pixelcircuit of FIG. 1.

First, a pixel is reset ahead of electric charge accumulation. Thereby,the reset line 9 and the transfer line 8 are set to a high level, sothat the reset transistor 3 and the transfer transistor 2 are in an ONstate. For example, this is an operation of transferring a power supplyvoltage of 3 V to the accumulation node 6 of the photodiode.

Thereby, a potential of the accumulation node 6 rises and the electronsaccumulated therein are extracted.

In a hole-accumulation diode (HAD) that has recently become mainstream,the accumulation node 6 is formed in an n-type buried diffusion layerinterposed between p-type layers, and all electrons are discharged, sothat it is in a full depletion state. An increase of the potential ofthe node 6 is also stopped at a point in time when all the electronshave been discharged, and its level becomes a predetermined level lowerthan the power supply voltage 3 V.

Thereafter, the transfer line 8 has a low level and the transfertransistor 2 is turned off, so that the accumulation node 6 is in thefloating state and new electric charge accumulation is started. Duringthe electric charge accumulation, the reset transistor 3 is normallyturned off.

In general, the above-described pixel reset operation is used as anelectronic shutter operation of a CMOS image sensor.

Next, an operation of reading the accumulated electric charge will bedescribed.

First, the row selection line 10 has a high level and the row selectiontransistor 5 is turned on, so that the amplifier transistor 4 of thepixel is connected to the vertical signal line 11.

Here, the vertical signal line 11 connected to the amplifier transistor4 and the constant current circuit 12 form a source follower circuit,and a potential Vf of the FD 7, which is its input, and a potential Vslof the vertical signal line 11, which is its output, have a linearrelationship close to a variation ratio of 1.

That is, if a current value of the constant current circuit 12 is i, thefollowing expression is ideally established.i=(1/2)*β*(Vf−Vth−Vsl)2, where β is a constant.  [Expression 1]

Here, (Vf−Vth−Vsl) becomes constant, and a variation of Vf is linearlyreflected in Vsl.

That is, the source follower circuit operates as an amplifier circuit inwhich a gain is about 1, and drives the vertical signal line 11according to a signal amount of the FD 7, which is an input node.

Here, the power supply voltage 3V is transferred to the FD 7 byswitching the reset line 9 to a high level and turning on the resettransistor 3.

Further, after the reset transistor 3 is turned off, first sensing ofthe potential Vsl of the vertical signal line 11 is performed by asensing circuit 13 constituted by a comparator, an analog/digitalconverter (ADC), or the like. This is the read of a reset signal.

Next, electrons accumulated in the accumulation node 6 flows into the FD7 by switching the transfer line 8 to the high level and turning on thetransfer transistor 2.

In this case, if the potential of the FD 7 is sufficiently deep, thatis, a high potential, all the electrons accumulated in the accumulationnode 6 are discharged to the FD 7, and the accumulation node 6 is in thefull depletion state.

Here, the transfer transistor 2 is turned off and second sensing of thepotential of the vertical signal line 11 is performed. This is the readof an accumulated signal.

A difference between the first sensing and the second sensing of theabove-described Vsl accurately reflects an electric charge amountaccumulated in the accumulation node 6 by exposure of the photodiode 1.

The CMOS imager digitalizes the difference and outputs the digitalizedresult to the outside as a pixel signal value. An electron accumulationtime of each pixel is a period of time between the reset operation andthe read operation described above, and accurately, is a period T1 untilthe transfer transistor 2 is turned off in the read after the transfertransistor 2 is reset and then turned off.

In general, in a CMOS type imager, accumulated electrons generated bythe photoelectric conversion element are converted into an analog signalof the vertical signal line 11 via the amplifier circuit for each pixel,and the analog signal is transferred to the sensing circuit 13.

Further, the analog signal is converted into a digital signal by theADC, and the digital signal is output outside a chip.

This is significantly different from a CCD type imager in whichaccumulated electrons themselves are vertically/horizontally transferredby a CCD transfer immediately before a chip output amplifier circuit.

SUMMARY OF INVENTION Technical Problem

Incidentally, because the photoelectrically converted electrons arecompletely transferred during a read operation in the above-describedpixel circuit, it is necessary to maintain a potential after thetransfer of the FD 7, which is an input node of the amplifier circuit118, to be higher than that of the photodiode 1 during full depletion.

However, a dynamic range of the potential of the FD 7 is limited, sothat there is a disadvantage in that it is not possible to sufficientlyincrease a potential variation amount ΔVf and increase an S/N ratio.

Further, because a saturated accumulated electric charge amount Qs ofthe photodiode corresponds to the number of donors within its diffusionlayer, the potential during the full depletion normally becomes deep (asa high potential) if the saturated accumulated electric charge amount Qsis increased. Thereby, the range of the potential variation amount ΔVfis in a narrower direction.

A problem of this transfer margin serves as a large limitation indesign.

The present invention is made in view of the above-mentioned issue, andaims to provide a pixel circuit, a solid-state image pickup device, anda camera system that can facilitate an electric charge transfer within apixel, improve an amount of accumulated electric charge or sensitivity,and improve image pickup capability.

Solution to Problem

According to a first aspect of the present invention, there is provideda pixel circuit including: a photoelectric conversion element; anamplifier circuit; and a transfer transistor for transferring electriccharge generated by the photoelectric conversion element to an inputnode of the amplifier circuit, wherein the transfer transistor hasfirst, second, and third field effect transistors integrated andconnected in series from the photoelectric conversion element to a sideof the amplifier circuit, the first and second field effect transistorshave gate electrodes to be simultaneously collectively driven, and athreshold voltage of the first field effect transistor is set to behigher than that of the second field effect transistor, and as the gateelectrodes are driven step by step, the electric charge generated by thephotoelectric conversion element and transferred via the first fieldeffect transistor is accumulated in a channel region of the second fieldeffect transistor, and the electric charge accumulated in the channelregion is transferred to an input of the amplifier circuit via the thirdfield effect transistor, and wherein the amplifier circuit drives asignal line, so that the accumulated electric charge is read.

According to a second aspect of the present invention, there is provideda pixel circuit including: a photoelectric conversion element; anamplifier circuit; and a transfer transistor for transferring electriccharge generated by the photoelectric conversion element to an inputnode of the amplifier circuit, wherein the transfer transistor has firstand second field effect transistors integrated and connected in seriesfrom the photoelectric conversion element to a side of the amplifiercircuit, the first and second field effect transistors have gateelectrodes to be simultaneously collectively driven, and a thresholdvoltage of the first field effect transistor is set to be higher thanthat of the second field effect transistor, and as the gate electrodesare driven step by step, a predetermined amount of electric chargegenerated by the photoelectric conversion element and transferred viathe first field effect transistor is accumulated in a channel region ofthe second field effect transistor, and the electric charge accumulatedin the channel region is transferred to an input of the amplifiercircuit, and wherein the amplifier circuit drives a signal line, so thatthe accumulated electric charge is read.

According to a third aspect of the present invention, there is provideda solid-state image pickup device including: a pixel section where aplurality of pixel circuits are arranged; and a pixel driving sectionfor reading a pixel signal by driving a pixel circuit of the pixelsection, wherein each pixel circuit has: a photoelectric conversionelement; an amplifier circuit; and a transfer transistor fortransferring electric charge generated by the photoelectric conversionelement to an input node of the amplifier circuit, wherein the transfertransistor has first, second, and third field effect transistorsintegrated and connected in series from the photoelectric conversionelement to a side of the amplifier circuit, the first and second fieldeffect transistors have gate electrodes to be simultaneouslycollectively driven, and a threshold voltage of the first field effecttransistor is set to be higher than that of the second field effecttransistor, and as the gate electrodes are driven by the pixel drivingsection step by step, the electric charge generated by the photoelectricconversion element and transferred via the first field effect transistoris accumulated in a channel region of the second field effecttransistor, and the electric charge accumulated in the channel region istransferred to an input of the amplifier circuit via the third fieldeffect transistor, and wherein the amplifier circuit drives a signalline, so that the accumulated electric charge is read.

According to a fourth aspect of the present invention, there is provideda solid-state image pickup device including: a pixel section where aplurality of pixel circuits are arranged; and a pixel driving sectionfor reading a pixel signal by driving a pixel circuit of the pixelsection, wherein each pixel circuit has: a photoelectric conversionelement; an amplifier circuit; and a transfer transistor fortransferring electric charge generated by the photoelectric conversionelement to an input node of the amplifier circuit, wherein the transfertransistor has first and second field effect transistors integrated andconnected in series from the photoelectric conversion element to a sideof the amplifier circuit, the first and second field effect transistorshave gate electrodes to be simultaneously collectively driven, and athreshold voltage of the first field effect transistor is set to behigher than that of the second field effect transistor, and as the gateelectrodes are driven by the pixel driving section step by step, apredetermined amount of electric charge generated by the photoelectricconversion element and transferred via the first field effect transistoris accumulated in a channel region of the second field effecttransistor, and the electric charge accumulated in the channel region istransferred to an input of the amplifier circuit, and wherein theamplifier circuit drives a signal line, so that the accumulated electriccharge is read.

According to a fifth aspect of the present invention, there is provideda camera system including: a solid-state image pickup device; an opticalsystem for forming an image of a subject on the image pickup device; anda signal processing circuit for processing an output image signal of theimage pickup device, wherein the solid-state image pickup device has: apixel section where a plurality of pixel circuits are arranged; and apixel driving section for reading a pixel signal by driving a pixelcircuit of the pixel section, wherein each pixel circuit has: aphotoelectric conversion element; an amplifier circuit; and a transfertransistor for transferring electric charge generated by thephotoelectric conversion element to an input node of the amplifiercircuit, wherein the transfer transistor has first, second, and thirdfield effect transistors integrated and connected in series from thephotoelectric conversion element to a side of the amplifier circuit, thefirst and second field effect transistors have gate electrodes to besimultaneously collectively driven, and a threshold voltage of the firstfield effect transistor is set to be higher than that of the secondfield effect transistor, and as the gate electrodes are driven by thepixel driving section step by step, the electric charge generated by thephotoelectric conversion element and transferred via the first fieldeffect transistor is accumulated in a channel region of the second fieldeffect transistor, and the electric charge accumulated in the channelregion is transferred to an input of the amplifier circuit via the thirdfield effect transistor, and wherein the amplifier circuit drives asignal line, so that the accumulated electric charge is read.

According to a sixth aspect of the present invention, there is provideda camera system including: a solid-state image pickup device; an opticalsystem for forming an image of a subject on the image pickup device; anda signal processing circuit for processing an output image signal of theimage pickup device, wherein the solid-state image pickup device has: apixel section where a plurality of pixel circuits are arranged; and apixel driving section for reading a pixel signal by driving a pixelcircuit of the pixel section, wherein each pixel circuit has: aphotoelectric conversion element; an amplifier circuit; and a transfertransistor for transferring electric charge generated by thephotoelectric conversion element to an input node of the amplifiercircuit, wherein the transfer transistor has first and second fieldeffect transistors integrated and connected in series from thephotoelectric conversion element to a side of the amplifier circuit, thefirst and second field effect transistors have gate electrodes to besimultaneously collectively driven, and a threshold voltage of the firstfield effect transistor is set to be higher than that of the secondfield effect transistor, and as the gate electrodes are driven by thepixel driving section step by step, a predetermined amount of electriccharge generated by the photoelectric conversion element and transferredvia the first field effect transistor is accumulated in a channel regionof the second field effect transistor, and the electric chargeaccumulated in the channel region is transferred to an input of theamplifier circuit, and wherein the amplifier circuit drives a signalline, so that the accumulated electric charge is read.

Advantageous Effects of Invention

According to the present invention, an electric charge transfer within apixel can be facilitated, an amount of accumulated electric charge orsensitivity can be improved, and image pickup capability can beimproved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of a pixel circuit.

FIG. 2 is a diagram showing a timing chart of the pixel circuit of FIG.1.

FIG. 3 is a diagram showing a configuration example of a CMOS imagesensor (solid-state image pickup device) adopting a pixel circuitaccording to an embodiment of the present invention.

FIG. 4 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a first embodiment of the present invention.

FIG. 5 is a diagram showing an equivalent circuit of a transfer circuitincluding a transfer transistor of the pixel circuit according to thefirst embodiment.

FIG. 6 is a diagram showing a cross-sectional structure example of thetransfer circuit of FIG. 5.

FIG. 7 is a diagram showing a potential change accompanying a readtransfer operation using the transfer circuit of the pixel circuitaccording to the first embodiment.

FIG. 8 is a diagram showing a timing chart when reset, electric chargeaccumulation, and read operations are performed in the pixel circuit ofFIG. 4.

FIG. 9 is a diagram showing an equivalent circuit of a transfer circuitincluding a transfer transistor of the pixel circuit of FIG. 1.

FIG. 10 is a diagram showing a potential change accompanying a readtransfer operation using the pixel circuit as shown in FIGS. 1 and 3.

FIG. 11 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a second embodiment of the present invention.

FIG. 12 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a third embodiment of the present invention.

FIG. 13 is a diagram showing a potential change accompanying a readtransfer operation using the pixel circuit according to the thirdembodiment.

FIG. 14 is a diagram showing a timing chart when reset, electric chargeaccumulation, and read operations are performed in the pixel circuit ofFIG. 12.

FIG. 15 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a fourth embodiment of the present invention.

FIG. 16 is a diagram showing a timing chart of an operation of a fifthembodiment.

FIG. 17 is a diagram showing a timing chart of an operation of a sixthembodiment.

FIG. 18 is a diagram showing a timing chart of a pixel operationadopting an intermediate retention mode and improving a large-capacityaccumulation operation of the fifth embodiment in a seventh embodiment.

FIG. 19 is a diagram showing a timing chart of a pixel operationadopting an intermediate retention mode and improving a large-capacityaccumulation operation of the sixth embodiment in an eighth embodiment.

FIG. 20 is a diagram showing a timing chart of an operation of a ninthembodiment in which a global shutter function is mounted in the firstembodiment.

FIG. 21 is a diagram showing a timing chart of an operation of a tenthembodiment in which a global shutter function is mounted in the thirdembodiment.

FIG. 22 is a first potential change diagram illustrating an example of awide dynamic timing range operation of an eleventh embodiment using aconfiguration of the first embodiment.

FIG. 23 is a second potential change diagram illustrating an example ofa wide dynamic range operation of the eleventh embodiment using theconfiguration of the first embodiment.

FIG. 24 is a diagram showing a timing chart of a wide dynamic rangecorrespondence operation according to the eleventh embodiment.

FIG. 25 is a diagram showing a timing chart of a wide dynamic rangeoperation of a twelfth embodiment using a configuration of the thirdembodiment.

FIG. 26 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a thirteenth embodiment of the present invention.

FIG. 27 is a diagram showing an equivalent circuit of a transfer circuitincluding a transfer transistor of a pixel circuit according to thethirteenth embodiment.

FIG. 28 is a diagram showing a cross-sectional structure example of thetransfer circuit of FIG. 27.

FIG. 29 is a diagram showing a potential change accompanying a readtransfer operation using a transfer circuit of the pixel circuitaccording to the thirteenth embodiment.

FIG. 30 is a diagram showing a pixel circuit of a CMOS imager accordingto a fourteenth embodiment of the present invention.

FIG. 31 is a diagram showing a configuration example of a transfercircuit according to a fifteenth embodiment in which a photodiode isreplaced with a MOS capacitor with respect to FIG. 6, which is thecross-sectional structure example corresponding to the first embodiment.

FIG. 32 is a diagram showing a cross-sectional structure example of atransfer circuit according to a sixteenth embodiment having across-sectional structure different from the transfer circuit of thefirst embodiment.

FIG. 33 is a diagram showing an example of a configuration of a camerasystem to which a solid-state image pickup device is applied accordingto an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the drawings, elements that have substantiallythe same function and structure are denoted with the same referencesigns, and repeated explanation is omitted.

Description will be given in the following order.

1. First Embodiment (First Configuration Example of Pixel Circuit)

2. Second Embodiment (Second Configuration Example of Pixel Circuit)

3. Third Embodiment (Third Configuration Example of Pixel Circuit)

4. Fourth Embodiment (Fourth Configuration Example of Pixel Circuit)

5. Fifth Embodiment (Electric Charge Accumulation Example Using DeepDepletion State)

6. Sixth Embodiment (Electric Charge Accumulation Example Using DeepDepletion State)

7. Seventh Embodiment (Intermediate Retention Mode)

8. Eighth Embodiment (Intermediate Retention Mode)

9. Ninth Embodiment (Global Shutter Function)

10. Tenth Embodiment (Global Shutter Function)

11. Eleventh Embodiment (Wide Dynamic Range Operation)

12. Twelfth Embodiment (Wide Dynamic Range Operation)

13. Thirteenth Embodiment (Fifth Configuration Example of Pixel Circuit)

14. Fourteenth Embodiment (Sixth Configuration Example of Pixel Circuit)

15. Fifteenth Embodiment (Another Cross-sectional Structure)

16. Sixteenth Embodiment (Another Cross-sectional Structure)

17. Seventeenth Embodiment (Camera System)

FIG. 3 is a diagram showing a configuration example of a CMOS imagesensor (solid-state image pickup device) adopting a pixel circuitaccording to an embodiment of the present invention.

A CMOS image sensor 100 has a pixel array section 110, a row selectioncircuit (Vdec) 120 as a pixel driving section, and a column read circuit(AFE) 130.

In the pixel array section 110, a plurality of pixel circuits 110A arearranged in a two-dimensional form (matrix form) of M rows×N columns.

According to this embodiment, the pixel circuit 110A basically includesa photoelectric conversion element, a transfer transistor, a resettransistor, an amplifier transistor, a row selection transistor, anaccumulation node, and FD (floating diffusion).

In this regard, the transfer transistor of the pixel circuit 110A isformed of at least first and second insulating gate type field effecttransistors (MOS transistors) integrated and connected in series asdescribed later. The first and second MOS transistors are formed of atransistor of a high threshold voltage Vth and a transistor of a lowthreshold voltage Vth having gate electrodes to be simultaneouslycollectively driven.

A specific configuration of the pixel circuit 110A will be describedlater.

A transfer line 140 (LTRG), a reset line 150 (LRST), and a row selectionline 160 (LSL) wired to the pixel array section 110 are wired in eachrow unit of a pixel arrangement as one set.

M control lines are provided for each of the transfer line 140 (LTRG),the reset line 150 (LRST), and the row selection line 160 (LSL).

The transfer line 140 (LTRG), the reset line 150 (LRST), and the rowselection line 160 (LSL) are driven by the row selection circuit 120.

The row selection circuit 120 controls an operation of a pixel arrangedin any row of the pixel array section 110. The row selection circuit 120controls the pixel circuit through the transfer line 140 (LTRG), thereset line 150 (LRST), and the row selection line 160 (LSL).

The column read circuit 130 receives data of a pixel row in which a readoperation is under control of the row selection circuit 120 via avertical signal line (LSGN) 170, and transfers the data to a rear-stagesignal processing circuit. A constant current circuit or a sensingcircuit is connected to the vertical signal line 170.

The column read circuit 130 includes a CDS circuit or an ADC(analog-digital converter).

Hereinafter, a specific configuration example of the pixel circuit ofthe CMOS image sensor 100 having the configuration as described abovewill be described.

<1. First Embodiment>

FIG. 4 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a first embodiment of the present invention.

The pixel circuit 110A (PX111) of one unit has a photodiode 111 as aphotoelectric conversion element, a transfer transistor 112, a resettransistor 113, an amplifier transistor 114, a row selection transistor115, an accumulation node 116, and FD 117.

An amplifier circuit 118 is formed of the amplifier transistor 114, andan input node of the amplifier circuit 118 is formed of the FD 117.

The transfer transistor 112 of the first embodiment is connected betweenthe photodiode 111 and the FD 117 as an output node.

The transfer transistor 112 is formed of at least a first MOS transistor1121, a second MOS transistor 1122, and a third MOS transistor 1123integrated and connected in series.

The first and second MOS transistors 1121 and 1122 are formed of atransistor of a high threshold voltage Vth and a transistor of a lowthreshold voltage Vth having gate electrodes to be driven by a drivingsignal simultaneously collectively applied.

The first MOS transistor 1121 is formed of a transistor of a highthreshold voltage HVth, and the second MOS transistor 1122 is formed ofa transistor of a low threshold voltage LVth.

The gate electrodes of the first and second MOS transistors 1121 and1122 are commonly connected to the transfer line 140, and a gateelectrode of the third MOS transistor 1123 is connected to a referencepotential, for example, a ground GND.

In this embodiment, the first, second, and third MOS transistors 1121,1122, and 1123 are formed of n-channel MOS (NMOS) transistors.

For example, the high threshold voltage HVth of the first NMOStransistor 1121 is set, for example, to 0 V, and the low thresholdvoltage LVth of the second NMOS transistor 1122 is set, for example, to−1.5 V.

A threshold voltage of the third NMOS transistor is set to −0.6 V.

The reset transistor 113 is connected between a power supply line andthe FD 117, and its gate electrode is connected to the reset line 150.

A gate of the amplifier transistor 114 is connected to the FD 117. Theamplifier transistor 114 is connected to the vertical signal line 170via the row selection transistor 115, and constitutes a source followerwith a constant current circuit 131 outside a pixel section.

A gate electrode of the row selection transistor 115 is connected to therow selection line 160. A source of the row selection transistor 115 isconnected to the vertical signal line 170.

The constant current circuit 131 and a sensing circuit 132 are connectedto the vertical signal line 170.

FIG. 5 is a diagram showing an equivalent circuit of a transfer circuitincluding the transfer transistor of the pixel circuit 110A according tothe first embodiment.

In a transfer circuit 200 of FIG. 5, reference numerals 201 and 202denote gate electrodes, reference numeral 203 denotes parasiticcapacitance, and reference numeral 118 denotes the amplifier circuit.The amplifier circuit 118 is formed of the amplifier transistor 114.

Electrons generated by photoelectric conversion in the photodiode 111are completely transferred to the FD 117, which is the input node of theamplifier circuit 118, via the first, second, and third NMOS transistors1121, 1122, and 1123 integrated and connected in series.

In the integrated first, second, and third NMOS transistors 1121, 1122,and 1123, channels are directly connected to each other, not via ann-type diffusion layer or the like.

As described above, a driving signal is simultaneously collectivelyapplied to gate electrodes 201 of the first and second NMOS transistors1121 and 1122.

The first MOS transistor 1121 has the high threshold voltage HVth, andthe second MOS transistor 1122 has the low threshold voltage LVth.

The FD 117, which is the input node, has the parasitic capacitance 203,and its potential variation amount ΔVf is as follows if an accumulatedelectric charge amount is Q and a parasitic capacitance value is Cf.ΔVf=Q/Cf  [Expression 2]

During a read operation, this displacement drives the vertical signalline 170 at a fixed gain via the amplifier circuit 118.

FIG. 6 is a diagram showing a cross-sectional structure example of thetransfer circuit of FIG. 5.

In the photodiode 111, a HAD structure in which the vicinity of asilicon surface in contact with an oxide film has a p-type is adopted.

Here, the photoelectrically converted electrons are initiallyaccumulated in an n-type diffusion node 204. The diffusion node 204corresponds to the accumulation node 116.

If a signal that turns on the first NMOS transistor 1121 is applied tothe gate electrode 201, the electrons are transferred to a channelregion of the second NMOS transistor 1122 via the first NMOS transistor1121, and accumulated in the channel region.

For example, an impurity profile of a channel portion is adjusted, sothat a threshold of the first NMOS transistor 1121 is set to be high anda threshold of the second NMOS transistor 1122 is set to be low.Thereby, a channel portion CH2 of the second NMOS transistor 1122 formsan electron accumulation well, and a channel portion CH1 of the firstNMOS transistor 1121 forms a potential wall of backflow prevention.

On the other hand, the third NMOS transistor 1123 is controlled by anindependent gate electrode 202.

A potential of a channel region of the third NMOS transistor 1123 is setto be shallower than that of the second NMOS transistor 1122 (as a highpotential) when an ON voltage is applied to the gate electrode 201.

The potential of the channel region is set to be deeper than that of thesecond NMOS transistor 1122 (as a low potential) when an OFF voltage isapplied to the gate electrode 201.

The gate electrode 202 of the third NMOS transistor 1123 may have afixed potential, and may also be connected to a power supply line, aground line, or the like if an impurity profile of a channel portion CH3of the third NMOS transistor 1123, or the like is appropriatelyadjusted.

A diffusion layer 205 is connected to an input of the amplifier circuit118, which is not shown in the cross-sectional view.

The third NMOS transistor 1123 functions as a separation transistor.

Here, the first NMOS transistor 1121 and the second NMOS transistor 1122are considered two individual transistors. However, they may also beconsidered a single NMOS transistor having a gradient in the impurityprofile of the channel portion if the gate electrodes are alsointegrally formed as shown in the figure.

In any case, it is functionally the same as two individual transistorsconnected in series. The present invention also includes theabove-described form.

FIGS. 7(A) to 7(D) are diagrams showing potential changes accompanying aread transfer operation using the transfer circuit of the pixel circuitaccording to the first embodiment.

In terms of the potential of each node in FIGS. 7(A) to 7(D), a positivepotential direction is shown on the lower side in the figure and anegative potential direction is shown on the upper side. Each nodeserves a well where electrons having negative electric charge areaccumulated, and the potential rises to the upper side, that is, in thenegative potential direction, with the well filled with the electrons.

[Step T11]

In step ST11 of FIG. 7(A), the diffusion node 204 of the photodiode 111is designed so that a potential bottom is about 2.0 V during fulldepletion in positive electric charge by a fixed number of donors. Here,it is filled with photoelectrically converted electrons up to asaturation state (about 0 V).

On the other hand, in the channel regions of the first NMOS transistor1121 and the second NMOS transistor 1122, potentials are respectivelymodulated in ranges of R11 and R12 according to a potential commonlyapplied to the gate electrodes of the two, for example, −1.5 V to 3 V.

On the other hand, the gate electrode 202 of the third NMOS transistor1123 as the separation transistor is connected to the ground GND, andthe potential of the channel is adjusted to about 0.6 V.

The diffusion layer 205 (the FD 117), which is the input node of theamplifier circuit 118, is reset to have a floating state of 3 V.

[Step ST12]

If the first NMOS transistor 1121 and the second NMOS transistor 1122are turned on in step ST12 of FIG. 7(B), electrons move as follows.

Electrons accumulated in the diffusion node 204 of the photodiode 111all move to the channel region of the second NMOS transistor 1122 viathe first NMOS transistor 1121.

That is, the electrons move to the channel region of the second NMOStransistor 1122 having a deep depletion state and are accumulated in ananalog state.

At this time, the potential of the channel region of the third NMOStransistor 1123 is shallower than that of the second NMOS transistor1122 (as a low potential), and forms a barrier between the second NMOStransistor 1122 and the diffusion layer 205 (the FD 117), which is theinput node of the amplifier circuit 118.

[Step ST13]

The gate electrodes are driven to turn off the first NMOS transistor1121 and the second NMOS transistor 1122 in step ST13 of FIG. 7(C), sothat the potentials of the channel regions are modulated in the negativepotential direction.

Here, the channel of the first NMOS transistor 1121 forms a potentialbarrier, and prevents accumulated electrons from flowing back to thediffusion node 204 of the photodiode 111.

A height of the barrier corresponds to a difference between thresholdsof two transistors of the first NMOS transistor 1121 and the second NMOStransistor 1122, for example, 1.5 V.

In the step in which the gate electrodes 201 of the first and secondNMOS transistors 1121 and 1122 have reached an appropriate intermediatevoltage, a state in which the accumulated electrons have been separatedfrom both the diffusion node 204 of the photodiode 111 and the FD 117,which is the input of the amplifier, is possible.

The gates may be directly driven at once until the next step, but it isalso possible to add a new function by temporarily retaining thisintermediate state as will be described later.

Further, if the potential of the channel region of the second NMOStransistor 1122 is modulated in the negative potential direction bycontinuously driving the gate from here, the electrons accumulatedtherein start to move to the diffusion layer 205 (the FD 117), which isthe input of the amplifier circuit 118.

[Step ST14]

If the first NMOS transistor 1121 and the second NMOS transistor 1122are completely turned off in step ST14 of FIG. 7(D), the potential ofthe channel region of the second NMOS transistor 1122 from which allaccumulated electrons have been discharged is as follows.

That is, the potential of the channel region of the second NMOStransistor 1122 exceeds the potential of the channel of the third NMOStransistor 1123.

In step ST11, a state is reached in which all electrons accumulated inthe photodiode 111 have moved to the diffusion layer 205 (the FD 117),which is the input node of the amplifier circuit 118.

Thereby, the amplifier circuit 118 drives the vertical signal line 170,and an accumulated signal is read.

If the above-described step-by-step transfer is used, it is unnecessaryto secure a potential difference between the diffusion node 204 of thephotodiode 111 having the full depletion state and the FD 117, which isthe input node of the amplifier circuit 118.

That is, in this example, a complete transfer is implemented even in astate in which the potential of the FD 117 filled with electrons isshallower than that of the diffusion node 204.

The operation of the transfer circuit of the pixel circuit 110Aaccording to the first embodiment has been mainly described above.

Next, electric charge accumulation and read operations of the pixelcircuit 110A of the first embodiment will be described.

FIGS. 8(A) to 8(D) are diagrams showing a timing chart when reset,electric charge accumulation, and read operations are performed in thepixel circuit of FIG. 4.

FIG. 8(A) shows a signal potential of the reset line 150, FIG. 8(B)shows a signal potential of the transfer line 140, FIG. 8(C) shows asignal potential of the row selection line 160, and FIG. 8(D) shows asignal potential of the vertical signal line 170.

As a characteristic operation of the pixel circuit 110A of the firstembodiment, the transfer of accumulated electrons of the photodiode 111to the FD 117 is performed in two steps in correspondence with drivingof the transfer line 140.

That is, if a level of the transfer line 140 rises from the low level tothe high level, the accumulated electrons are transferred from thediffusion node 204 of the photodiode 111 to the channel region of thesecond NMOS transistor 1122 as shown in step ST12 of FIG. 7(B).

Further, when the level of the transfer line 140 returns from the highlevel to the low level, the electrons of the channel region aretransferred to the FD 117, which is the input node of the amplifiercircuit 118, as shown in step ST 14 of FIG. 7(D).

For example, the reset line 150 has the high level during reset, so thatthe FD 117, which is the input node of the amplifier circuit 118, isconnected to a reset level potential (power supply voltage 3V).

On the other hand, at a point in time when the level of the transferline 140 has risen from the low level to the high level and has furtherfallen from the high level to the low level, the electrons accumulatedin the photodiode 111 are transferred to the diffusion layer 205 anddrawn to the reset level.

At this point in time, a new electron accumulation period T2 starts.

A reset pulse of the reset line 150 falls to the low level after waitingfor the level of the transfer line 140 to fall to the low level.

Likewise, during a read operation also, the electrons accumulated in thephotodiode 111 are transferred to the diffusion layer 205 at a point intime when the level of the transfer line 140 has risen from the lowlevel to the high level and further fallen from the high level to thelow level.

Therefore, the vertical signal line 170 is driven by the accumulatedsignal via the amplifier circuit 118 at a point in time when the levelof the transfer line 140 has returned from the high level to the lowlevel. At this point in time, the accumulation period T2 also ends.

The pixel circuit 110A according to the above-described first embodimentcan facilitate an electric charge transfer within a pixel of a CMOSimage sensor, improve an accumulated electric charge amount orsensitivity, and improve image pickup capability.

Here, processing of a transfer circuit system of the pixel circuit ofFIG. 1 for a comparison with the pixel circuit 110A according to theabove-described first embodiment will be described.

FIG. 9 is a diagram showing an equivalent circuit of the transfercircuit including the transfer transistor of the pixel circuit PX1 ofFIG. 1.

In the transfer circuit TX1 of FIG. 9, GT1 denotes a gate electrode, C1denotes parasitic capacitance, and reference numeral 14 denotes theamplifier circuit. The amplifier circuit 14 is formed of the amplifiertransistor 4.

In the transfer circuit TX1, electrons generated by photoelectricconversion in the photodiode 1 are accumulated in the accumulation node6, which is a diffusion layer node of the photodiode 1.

During a read operation, the electrons are completely transferred to theFD 7, which is the input node of the amplifier circuit 14, via thetransfer transistor 2.

If the FD 7, which is the input node, has the parasitic capacitance C1,an accumulated electric charge amount is Q, and a parasitic capacitancevalue is Cf, its potential variation amount ΔVf is given as {ΔVf=Q/Cf}as described above.

The amplifier circuit 14 uses an NMOS transistor like the normalamplifier transistor 4, but generates unique random noise Nr.

Therefore, if its gain is G, an S/N ratio of an accumulated signaloccurring in the vertical signal line as an output is {G·ΔVf/Nr}.

Because the gain G or the random noise Nr are substantially fixed if theconfiguration of the amplifier circuit 14 is decided, the magnitude ofthe potential variation amount ΔVf directly affects image pickupperformance.

FIGS. 10(A) to 10(D) are diagrams showing potential changes accompanyinga read transfer operation using the pixel circuit as shown in FIGS. 1and 3.

As in FIGS. 7(A) to 7(D), in this case also, in terms of the potentialof each node, a positive potential direction is shown on the lower sidein the figure and a negative potential direction is shown on the upperside.

Each node serves a well where electrons having negative electric chargeare accumulated, and the potential rises to the upper side, that is, inthe negative potential direction, with the well filled with theelectrons.

[Step ST1]

In step ST1 of FIG. 10(A), the accumulation node 6, which is thediffusion node of the photodiode 1, is designed so that a potentialbottom is about 1.5 V during full depletion in positive electric chargeby a fixed number of donors. Here, it is filled with photoelectricallyconverted electrons up to a saturation state (about 0 V).

On the other hand, the channel region of the transfer transistor 2 ismodulated in a range of R1 according to a potential applied to the gateelectrode, for example, 1 V to 3 V.

The FD 7, which is the input node of the amplifier circuit 14, is resetto have a floating state of 3 V.

[Step ST2]

If the transfer transistor 2 is turned on in step ST2 of FIG. 10(B),electrons move as follows.

That is, if the transfer transistor 2 is turned on, the electrons movein a state in which all the electrons accumulated in the accumulationnode 6, which is the diffusion node of the photodiode 1, have beendistributed to the channel region of the transfer transistor 2 and theFD 7, which is the input node of the amplifier circuit 14.

[Step ST3]

If the potential of the channel region rises with a potential increasein the gate electrode in order to turn off the transfer transistor 2 instep ST3 of FIG. 10(C), the electrons accumulated therein move to the FD7, which is the input node of the amplifier circuit 14.

[Step ST4]

In step ST4 of FIG. 10(D), a state is reached in which all electronsaccumulated in the photodiode 1 in step ST1 have moved to the FD 7,which is the input node of the amplifier circuit 14, in a state in whichthe transfer transistor 2 has been turned off. Thereby, the amplifiercircuit 14 drives the vertical signal line 11, and an accumulated signalis read.

As described above, it is necessary for the pixel circuit PX1 of FIG. 1to secure a potential difference M1 between the accumulation node 6 ofthe photodiode 1 having a full depletion state and the FD 7, which isthe input node of the amplifier circuit 14, in order to implementcomplete electron movement.

On the other hand, if the potential difference is not sufficientlysecured, the electrons accumulated in the channel region of the transfertransistor 2 flow back to the photodiode 1, and an accumulated electronamount of the photodiode 1 is not linearly reflected in a read signal.

In order to completely transfer electrons photoelectrically convertedduring a read operation as described above, it is necessary to maintainthe potential after the transfer of the FD 7, which is the input node ofthe amplifier circuit 14, as a potential higher than that of thephotodiode 1 during full depletion.

However, the pixel circuit PX1 of FIG. 1 has a disadvantage in that adynamic range of the potential of the FD 7 is limited, ΔVf cannot besufficiently increased, and the S/N ratio cannot be increased.

For example, in FIG. 10, ΔVf has a limit of (3.0 V−1.5 V), and isfurther reduced by a potential difference of a transfer margin.

Further, because a saturated accumulated electric charge amount Qs ofthe photodiode 1 corresponds to the number of donors within itsdiffusion layer, the potential during the full depletion becomes deep(as a high potential) if Qs is normally increased. Thereby, a range ofΔVf is in a narrower direction.

A problem of this transfer margin serves as a large limitation indesign.

On the other hand, the pixel circuit 110A of the first embodiment adoptsa transfer by integrated series transistors for the transfer ofelectrons from the photodiode within the pixel to the amplifier circuit.Specifically, in the pixel circuit 110A, an intermediate transfer nodein which potential modulation is possible is formed in a channel portionof a MOS transistor, and accumulated electrons are transferred step bystep from the photodiode 111 to the amplifier circuit 118 via theintermediate node.

Therefore, the pixel circuit 110A of the first embodiment can releasethe above-described potential limitation for the transfer, and henceimprove a dynamic range of a signal by increasing the saturatedaccumulated electric charge amount Qs or reducing parasitic capacitanceof an amplifier input portion.

Further, for example, the pixel circuit 110A can accumulate electronsphotoelectrically converted during exposure in the channel region of theMOS transistor separately formed, not within the photodiode, andcompletely transfer the accumulated electrons from the channel region tothe amplifier circuit during a read operation.

Therefore, the pixel circuit 110A can improve exposure sensitivity andalso significantly improve the saturated accumulated electric chargeamount Qs.

In the pixel circuit 110A, an electron (electric charge) transfer isperformed only inside the pixel, and an analog signal of low impedanceor a digital signal is transferred after driving of the vertical signalline by the amplifier circuit.

Therefore, it is possible to implement a high-speed, low-powerconsumption imager without a problem of M smear or transfer leak.

<2. Second Embodiment>

FIG. 11 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a second embodiment of the present invention.

A difference between a pixel circuit 110B according to the secondembodiment and the pixel circuit 110A according to the first embodimentis as follows.

In the pixel circuit 110B according to the second embodiment, aplurality of pixels, for example, two pixels PXL110 a and PXL110 b,respectively having a unique photodiode 111 and a unique transfercircuit 112, share FD 117 and an amplifier transistor 114 forming anamplifier circuit.

In the pixel circuit 110B, the plurality of pixels PXL110 a and PXL110 balso share a reset transistor 113 and a row selection transistor 115.

In transfer transistors 112 a and 112 b of the respective pixels PXL110a and PXL110 b, shared gate electrodes of first and second NMOStransistors are respectively connected to different transfer lines 140 aand 140 b.

Incidentally, gate electrodes of third MOS transistors 1123 of thetransfer transistors 112 a and 112 b of the respective pixels PXL110 aand PXL110 b are respectively grounded.

In the pixel circuit 110B, electrons accumulated in respectivephotodiodes 111 a and 111 b are transferred to the FD117 (an input nodeof the amplifier circuit) at individual timings according to therespectively independent transfer lines 140 a and 140 b.

The sharing of the amplifier circuit can reduce an effective size of thepixel, but parasitic capacitance of the FD 117 also increases when thenumber of sharing pixels increases.

Therefore, it is preferable that the number of sharing pixels be equalto or greater than 2 and equal to or less than 16.

The second embodiment can have the same advantageous effects as theabove-described first embodiment.

<3. Third Embodiment>

FIG. 12 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a third embodiment of the present invention.

A difference between a pixel circuit 110C according to the thirdembodiment and the pixel circuit 110A according to the first embodimentis as follows.

In the pixel circuit 110C of the third embodiment, a gate electrode 202of a third NMOS transistor 1123, which has a fixed potential in thefirst embodiment, is subsidiarily driven by a row selection circuit 120,which is a peripheral circuit.

Specifically, a shared gate electrode 201 of first and second NMOStransistors 1121 and 1122 is connected to a first transfer line 141, andthe gate electrode 202 of the third NMOS transistor 1123 is connected toa second transfer line (separation line) 142.

Thereby, as the transfer via the third NMOS transistor 1123 can befacilitated, a driving range at the side of the gate electrode 201 canbe narrowed.

While there is a disadvantage in an area when the number of drivingwirings is increased by 1, there is an advantage in voltage resistanceor reliability when the driving range of the first transfer line 141 canbe narrowed.

FIGS. 13(A) to 13(D) are diagrams showing potential changes accompanyingthe read transfer operation using the pixel circuit 110C according tothe third embodiment.

[Step ST21]

In step ST21 of FIG. 13(A), a diffusion node 204 of a photodiode 111 isdesigned so that a potential bottom is about 2.0 V during full depletionin positive electric charge by a fixed number of donors. Here, it isfilled with photoelectrically converted electrons up to a saturationstate (about 0 V).

On the other hand, in channel regions of the first NMOS transistor 1121and the second NMOS transistor 1122, potentials are respectivelymodulated in ranges of R13 and R14 according to a potential commonlyapplied to gate electrodes of the two, for example, −0.5 V to 3 V.

On the other hand, in a channel region of the third NMOS transistor 1123as a separation transistor, a potential uniquely applied to its gateelectrode is modulated in a range of R15, for example, according to 0 Vto 3 V.

A diffusion layer 205 (FD 117), which is an input node of an amplifiercircuit 118, is reset to have a floating state of 3 V.

[Step ST22]

If the first NMOS transistor 1121 and the second NMOS transistor 1122are turned on in step ST22 of FIG. 13(B), an electron transfer isperformed as follows.

That is, electrons accumulated in the diffusion node 204 of thephotodiode 111 all move to the channel region of the second NMOStransistor 1122 via the first NMOS transistor 1121.

That is, the electrons move to the channel region of the second NMOStransistor 1122 having a deep depletion state and are accumulated in ananalog state.

At this time, the potential of the channel region of the third NMOStransistor 1123 is shallower than that of the second NMOS transistor1122 (as a low potential), and forms a barrier between the second NMOStransistor 1122 and the diffusion layer 205 (the FD 117), which is theinput node of the amplifier circuit 118.

[Step ST23]

If the first NMOS transistor 1121 and the second NMOS transistor 1122are turned off again in step ST23 of FIG. 13(C), the potentials of thechannel regions are modulated in the negative potential direction.

Here, a channel of the first NMOS transistor 1121 forms a potentialbarrier, and prevents accumulated electrons from flowing back to thediffusion node 204 of the photodiode 111.

A height of the barrier corresponds to a difference between thresholdsof two transistors of the first NMOS transistor 1121 and the second NMOStransistor 1122, for example, 1.5 V.

In this step, a state in which the accumulated electrons have beenseparated from both the diffusion node 204 of the photodiode 111 and thediffusion layer 205 (the FD 117), which is the input of the amplifiercircuit 118, is possible.

It may proceed to the next step at once by directly or simultaneouslydriving the gate of the third NMOS transistor 1123, but it is alsopossible to add a new function by temporarily retaining thisintermediate state as will be described later.

[Step ST24]

If the gate electrode 202 of the third NMOS transistor 1123 is driventhrough the second transfer line 142 as the separation line in step ST24of FIG. 13(D) and the third NMOS transistor 1123 is turned on, theaccumulated electrons flow into the FD 117, which is the input node ofthe amplifier circuit 118.

Further, at a point in time when the third NMOS transistor 1123 has beenturned off, a state is reached in which all the accumulated electronshave moved to the diffusion layer 205 (the FD 117), which is the inputnode of the amplifier circuit 118.

Thereby, the amplifier drives the vertical signal line and anaccumulated signal is read.

As described above, the transfer from the channel of the second NMOStransistor 1122 to the FD 117, which is the input node of the amplifiercircuit 118, in the third embodiment is performed as follows.

It is implemented by combining both OFF driving of the shared gateelectrode 201 of the first and second NMOS transistors 1121 and 1122 ofstep ST23 and auxiliary ON/OFF driving of the gate electrode 202 of thethird NMOS transistor 1123 of step ST24.

If the above-described step-by-step transfer is used, it is unnecessaryto secure a potential difference between the diffusion node 204 of thephotodiode 111 having the full depletion state and the FD 117, which isthe input node of the amplifier circuit 118.

That is, in this example, a complete transfer is implemented even in astate in which the potential of the FD 117 filled with electrons isshallower than that of the diffusion node 204.

The transfer operation of the pixel circuit 110C according to the thirdembodiment has been mainly described above.

Next, electric charge accumulation and read operations of the pixelcircuit 110C of the third embodiment will be described.

FIGS. 14(A) to 14(E) are diagrams showing a timing chart when the reset,electric charge accumulation, and read operations are performed in thepixel circuit of FIG. 12.

FIG. 14(A) shows a signal potential of a reset line 150, FIG. 14(B)shows a signal potential of the first transfer line 141, and FIG. 14(C)shows a signal potential of the second transfer line 142. FIG. 14(D)shows a signal potential of a row selection line 160, and FIG. 14(E)shows a signal potential of a vertical signal line 170.

A main difference in the operation between the third embodiment and thefirst embodiment shown in FIG. 8 is as follows.

In the third embodiment, an ON/OFF pulse of the second transfer line142, which drives the third NMOS transistor 1123, is added to assist thetransfer of accumulated electrons when the first and second NMOStransistors 1121 and 1122 are turned off by the first transfer line 141.

That is, if a level of the first transfer line 141 rises from the lowlevel to the high level, the accumulated electrons are transferred fromthe diffusion node 204 of the photodiode 111 to the channel region ofthe second NMOS transistor 1122 as shown in step ST22 of FIG. 13(B).

Further, if the level of the first transfer line 141 returns from thehigh level to the low level and the second transfer line 142 as theseparation line has the high level at substantially the same time, theelectrons move as follows.

As shown in step ST24 of FIG. 13(D), the third NMOS transistor 1123 asthe separation transistor is conductive and the accumulated electronsflow into the FD 117, which is the input node of the amplifier circuit118.

Ultimately, if the level of the second transfer line 142 as theseparation line falls to the low level, a complete transfer of electronsto the FD 117, which is the input node of the amplifier circuit 118, iscompleted.

For example, the reset line 150 has the high level during reset, so thatthe FD 117, which is the input node of the amplifier circuit, isconnected to a reset level potential (power supply voltage 3 V).

On the other hand, the level of the first transfer line 141 rises fromthe low level to the high level, so that extra electrons accumulated inthe photodiode 111 are transferred to a channel portion CH2 of thesecond NMOS transistor 1122 via the first NMOS transistor 1121.

Further, the level of the first transfer line 141 falls from the highlevel to the low level and the second transfer line 142 as theseparation line has the high level at substantially the same time, sothat the third NMOS transistor 1123, which is the separation transistor,is conductive.

Ultimately, if the level of the second transfer line 142 as theseparation line falls to the low level, the accumulated electrons arecompletely transferred to the FD 117 and drawn to the reset level.

At this point in time, a new electron accumulation period T3 starts.More precisely, T3 starts at a point in time when the level of the firsttransfer line 141 has fallen from the high level to the low level.

Likewise, the level of the first transfer line 141 first rises from thelow level to the high level during a read operation also, so that theelectrons accumulated in the photodiode 111 are transferred to thechannel portion of the second MOS transistor 1122 via the first NMOStransistor 1121.

Further, if the level of the first transfer line 141 falls from the highlevel to the low level and the level of the second transfer line 142 asthe separation line rises to the high level at substantially the sametime and further ultimately falls to the low level, the electrons arecompletely transferred to the FD 117, which is the input node of theamplifier circuit 118.

At this point in time, the accumulation period T3 ends.

The timing that the second transfer line 142 as the separation line isturned on at the high level may be before/after the timing that thefirst transfer line 141 is turned off at the low level.

If the amplifier transistor 114 has been turned on through the FD 117before the first transfer line 141 has an OFF level, it moves to stepST14 of FIG. 13(D) by skipping the states of step ST22 of FIG. 13(B) tostep ST23 of FIG. 13(C).

However, a complete transfer can be implemented if the second transferline 142 as the separation line has the OFF level after the firsttransfer line 141 has the OFF level.

The third embodiment can improve voltage resistance or reliability inaddition to the advantageous effects of the first embodiment.

<4. Fourth Embodiment>

FIG. 15 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a fourth embodiment of the present invention.

A difference between a pixel circuit 110D according to the fourthembodiment and the pixel circuit 110C according to the third embodimentis as follows.

In the pixel circuit 110D according to the fourth embodiment, aplurality of pixels, for example, two pixels PXL110 a and PXL110 b,respectively having a unique photodiode 111 and a unique transfercircuit 112, share FD 117 and an amplifier transistor 114 forming anamplifier circuit.

In the pixel circuit 110D, the plurality of pixels PXL110 a and PXL110 balso share a reset transistor 113 and a row selection transistor 115.

In transfer transistors 112 a and 112 b of the respective pixels PXL110a and PXL110 b, shared gate electrodes of first and second NMOStransistors are respectively connected to different first transfer lines141 a and 141 b.

Gate electrodes of third MOS transistors 1123 of the transfertransistors 112 a and 112 b of the respective pixels PXL110 a and PXL110b are respectively connected to second transfer lines 142 a and 142 b asseparation lines.

In the pixel circuit 110D, electrons accumulated in respectivephotodiodes 111 a and 111 b are transferred to the FDF 117 at individualtimings according to the respectively independent first transfer lines141 a and 141 b and the second transfer lines 142 a and 142 b as theseparation lines.

The sharing of the amplifier circuit can reduce an effective size of thepixel, but parasitic capacitance of the FD 117 also increases when thenumber of sharing pixels increases.

Therefore, it is preferable that the number of sharing pixels be equalto or greater than 2 and equal to or less than 16.

The fourth embodiment can have the same advantageous effects as theabove-described first embodiment.

Next, an application operation, which makes it possible to performlarge-capacity accumulation by utilizing the configuration of the pixelcircuit according to the embodiment of the present invention, will bedescribed.

The large-capacity accumulation operation is applicable to any circuitconfiguration of the above-described first to fourth embodiments, andwill be described below as fifth and sixth embodiments.

<5. Fifth Embodiment>

In a fifth embodiment of the present invention, electric chargeaccumulation is applied using the pixel circuit configuration of FIG. 4and the deep depletion state of the second NMOS transistor 1122 adoptedin the first embodiment.

Specifically, electrons accumulated in the diffusion node 204, which isthe diffusion layer of the photodiode 111, are transferred to thechannel portion of the second NMOS transistor 1122 and accumulated inthe channel portion during an accumulation period.

That is, during the accumulation period of a pixel, the gate electrode201 is maintained at a level of an ON state so that the first and secondNMOS transistors 1121 and 1122 are maintained in the ON state.

Electrons photoelectrically converted by the photodiode 111 are directlytransferred to the channel portion CH2 of the second NMOS transistor1122 via the first NMOS transistor 1121, and accumulated in the channelportion CH2.

At a point in time when accumulation is completed and read is performed,the gate electrode 201 is driven to turn off the first and second NMOStransistors 1121 and 1122. Thereby, the accumulated electrons aretransferred to the FD 117, which is the input node of the amplifiercircuit 118, via the third NMOS transistor 1123.

FIGS. 16(A) to 16(D) are diagrams showing a timing chart of an operationof the fifth embodiment.

FIG. 16(A) shows a signal potential of the reset line 150, FIG. 16(B)shows a signal potential of the transfer line 140, FIG. 16(C) shows asignal potential of the row selection line 160, and FIG. 16(D) shows asignal potential of the vertical signal line 170.

In the fifth embodiment, a pixel circuit is the same as shown in FIG. 4,and details and a cross-sectional configuration of a transfer circuitare the same as shown in FIGS. 5 and 6.

The same reset as in FIGS. 8(A) to 8(D) is performed. After newaccumulation starts, the transfer line 140 has the high-level stateagain, and is maintained in the high-level state during an accumulationperiod T4.

During this period, electrons photoelectrically converted by thephotodiode 111 are not accumulated within the diffusion layer, but aredirectly transferred to the channel region of the second NMOS transistor1122 and accumulated in the channel region.

During a read operation, first, the row selection line 160 has the highlevel, and an output of the amplifier circuit is connected to thevertical signal line 170.

Further, the FD 117, which is the input node of the amplifier circuit118, is reset by a pulse directed to the reset line 150, and the FD 117is connected to a power supply voltage source, so that the reset levelis read.

Next, the transfer line 140 has a transition from the high level to thelow level.

Thereby, the electrons accumulated in the channel region of the secondNMOS transistor 1122 are transferred to the FD 117, which is the inputnode of the amplifier circuit 118, and an accumulated signal is read.

The accumulation period T4 also ends with the transition of the transferline 140.

A potential change of this embodiment is based on FIGS. 7(A) to 7(D),but the duration of electron accumulation has the state of step ST12 ofFIG. 7(B), not step ST11 of FIG. 7(A).

In the accumulation period, the first NMOS transistor 1121 and thesecond NMOS transistor 1122 are maintained in the ON state. Then,electrons photoelectrically converted by the photodiode 111 andcollected in the diffusion node 204 directly move to the channel regionof the second NMOS transistor 1122 via the first NMOS transistor 1121.

That is, the electrons move to the channel region of the second NMOStransistor 1122 having the deep depletion state, and are accumulated inan analog state.

The transfer of electrons from the second NMOS transistor 1122 to thediffusion layer 205 during a read operation is the same as the processof steps ST13 and ST14 of FIGS. 7(C) and 7(D).

As described above, during the accumulation period, the electronsphotoelectrically converted in the state of step ST12 of FIG. 7(B) areall accumulated in the channel region of the second NMOS transistor1122, and there is no saturation until its potential well is full.

Therefore, it is possible to accumulate a larger number of electronsthan in accumulation directed to a normal photodiode if accumulationcapacity is sufficiently increased in the deep depletion state of thesecond NMOS transistor 1122.

Further, during this period, the photodiode 111 is constantly maintainedin the same full depletion state. Therefore, sensitivity or linearity ofan accumulation time and an accumulated signal is improved.

In general, in terms of electron/hole pairs generated by light incidencedirected to a photodiode, holes generated inside a depletion layer aredrawn by its internal electric field and rapidly discharged to asubstrate.

However, if electrons are accumulated inside the photodiode, theinternal electric field is mitigated, hole discharge capability isreduced, and electrons and holes are easily recombined.

Thereby, there is a problem in that sensitivity is gradually reduced.

On the other hand, this problem does not occur in the fifth embodiment.

Because there is no problem even when a saturated accumulated electriccharge amount of the photodiode itself is little, it is possible tosuppress a dark current or white spot from occurring by reducing animpurity concentration of the diffusion layer.

In this case, the potential formed in the diffusion node 204 of thephotodiode 111 can be shallow during reset. Therefore, the modulationranges R11 and R12 of the channel regions of the first NMOS transistor1121 and the second NMOS transistor 1122 can also be decreased andsecurement of reliability of voltage resistance or the like isfacilitated.

As described above, it is possible to improve all basic performances ofan image pickup device for an accumulated electric charge amount,sensitivity, and a white spot.

<6. Sixth Embodiment>

A sixth embodiment in which the same concept is applied to the thirdembodiment will be described.

FIG. 17(A) shows a signal potential of the reset line 150, FIG. 17(B)shows a signal potential of the first transfer line 141, and FIG. 17(C)shows a signal potential of the second transfer line (separation line)142. FIG. 17(D) shows a signal potential of the row selection line 160,and FIG. 17(E) shows a signal potential of the vertical signal line 170.

In the sixth embodiment, a pixel circuit is the same as that of FIG. 12,and a potential change of the transfer is based on FIGS. 13(A) to 13(D).

The same reset as in FIGS. 14(A) to 14(E) is performed. After newaccumulation starts, the first transfer line 141 is in the high-levelstate again, and is maintained in the high-level state during anaccumulation period T5.

During this period, electrons photoelectrically converted by thephotodiode 111 are not accumulated within the diffusion layer, but aredirectly transferred to the channel region of the second NMOS transistor1122 and accumulated in the channel region.

That is, the state of step ST22 of FIG. 13(B) of the potential diagramsis retained.

During a read operation, first, the row selection line 160 has the highlevel, and an output of the amplifier circuit 118 is connected to thevertical signal line 170.

Further, the FD 117, which is the input node of the amplifier circuit118, is reset by a pulse directed to the reset line 150, and the FD 117is connected to a power supply voltage source, so that a reset level isread.

Next, the first transfer line 141 has a transition from the high levelto the low level, and further a pulse is also applied to the secondtransfer line 142 as the separation line.

Thereby, electrons accumulated in the channel region of the second NMOStransistor 1122 are transferred to the FD 117, which is the input nodeof the amplifier circuit 118, and an accumulated signal is read.

The accumulation period T5 also ends with the transition of the firsttransfer line 141.

As described above, during the accumulation period, the electronsphotoelectrically converted in the state of step ST22 of FIG. 13(B) areall accumulated in the channel region of the second NMOS transistor1122, and there is no saturation until its potential well is full.

Therefore, it is possible to accumulate a larger number of electronsthan in accumulation directed to a normal photodiode if accumulationcapacity is sufficiently increased in the deep depletion state of thesecond NMOS transistor 1122.

Incidentally, the intermediate state of step ST13 in the potentialchange of FIG. 7 showing the first embodiment is as follows.

Electrons accumulated in the channel region of the second NMOStransistor 1122, which is the intermediate accumulation node, areseparated from the FD 117, which is in the amplifier input, as well asfrom the diffusion node 204 of the photodiode 111.

That is, electrons newly photoelectrically converted in the diffusionnode 204 of the photodiode 111 do not flow into the intermediateaccumulation node and electrons accumulated in the intermediateaccumulation node do not flow into the FD 117.

In the first embodiment, the shared gate of the first and second NMOStransistors 1121 and 1122 is driven in three values, and theabove-described state is implemented at its intermediate voltage, sothat the intermediate state can be retained during a fixed period.

Likewise, the potential change of FIG. 13 showing the second embodimentin the intermediate state of step ST23 is as follows.

The electrons accumulated in the channel region of the second NMOStransistor 1122, which is the intermediate accumulation node, areseparated from the FD 117, which is in the amplifier input, as well asfrom the diffusion node 204 of the photodiode 111.

In this case, it is possible to retain the intermediate state during afixed period by turning off all the first and second NMOS transistors1121 and 1122 and the third NMOS transistor 1123 by a gate electrodedriving operation.

Various additional functions can be implemented if accumulated electronsreceived from the photodiode 111 are kept during a fixed period in theintermediate accumulation node in which potential modulation ispossible.

More specifically, the fixed period is, for example, a period equal toor greater than a minimum accumulation period, or a period equal to orgreater than a period taken to read one row.

Hereinafter, three functions of large-capacity accumulation, a globalshutter, and a wide dynamic range will be sequentially described inseventh to fourteenth embodiments.

All of the seventh to twelfth embodiments can be equally performed usingeach configuration of the above-described first to fourth embodiments.

<7. Seventh Embodiment>

FIGS. 18(A) to 18(D) are diagrams showing a timing chart of a pixeloperation adopting the above-described intermediate retention mode andimproving the large-capacity accumulation operation of the fifthembodiment in a seventh embodiment.

FIG. 18(A) shows a signal potential of the reset line 150, FIG. 18(B)shows a signal potential of the transfer line 140, FIG. 18(C) shows asignal potential of the row selection line 160, and FIG. 18(D) shows asignal potential of the vertical signal line 170.

In the seventh embodiment, a pixel circuit is the same as shown in FIG.4, and details and a cross-sectional configuration of a transfer circuitare the same as shown in FIGS. 5 and 6.

If the first and second NMOS transistors 1121 and 1122 are continuouslyopened via the gate electrode 201, there is a tendency for a potentialof the photodiode 111 of FIG. 6 around the gate to rise and a darkcurrent to increase.

In view of the above-described problem, the seventh embodiment, which isan improved example, suppresses the dark current from being increased bydriving the gate electrode 201 and intermittently turning on the firstand second NMOS transistors 1121 and 1122.

That is, first, new accumulation is started by performing the same resetoperation as that of FIG. 8 or 16. Thereafter, during an accumulationperiod T6, electrons are transferred from the photodiode 111 to theintermediate accumulation node by time division by causing the first andsecond NMOS transistors 1121 and 1122 to intermittently have the highlevel without being continuously opened via the gate electrode 201.

During the accumulation period other than a transfer time after aninitial transfer, the gate electrode 201 is kept at the intermediatepotential, and the intermediate state of step ST13 of FIG. 7(C) ismaintained in the intermediate accumulation node.

The gate electrode 201 has the high level again during a read operation,and electrons remaining in the photodiode 111 are transferred to theintermediate accumulation node. Ultimately, the gate electrode 201 hasthe low level, and accumulated electrons of the intermediateaccumulation node are together collectively transferred to the FD 117,which is the amplifier input.

Normally, because all photoelectrically converted electrons areaccumulated in the photodiode 111 after the reset operation, itssaturated accumulated electric charge amount Qs decides a dynamic rangeof a pixel.

However, if the accumulation capacity of the intermediate accumulationnode having the channel portion of the second NMOS transistor 1122 issufficiently large, it is possible to accumulate a larger amount ofelectric charge than normal by transferring the accumulated electriccharge of the photodiode 111 to the intermediate accumulation node aplurality of times by time division.

Because a period in which the first and second NMOS transistors 1121 and1122 are turned on through the gate electrode 201 can be sufficientlyreduced as compared to the accumulation period, it is also possible toprevent a dark current from being increased.

<8. Eighth Embodiment>

FIGS. 19(A) to 19(E) are diagrams showing a timing chart of a pixeloperation adopting the same intermediate retention mode as that of theseventh embodiment and improving the large-capacity accumulationoperation of the sixth embodiment in an eighth embodiment.

FIG. 19(A) shows a signal potential of the reset line 150, FIG. 19(B)shows a signal potential of the first transfer line 141, and FIG. 19(C)shows a signal potential of the second transfer line (separation line)142. FIG. 19(D) shows a signal potential of the row selection line 160,and FIG. 19(E) shows a signal potential of the vertical signal line 170.

In the eighth embodiment, a pixel circuit is the same as that of FIG.12, and a potential change of the transfer is based on FIGS. 13(A) to13(D).

In the eighth embodiment, which is an improved example, new accumulationis started by performing the same reset as that of FIG. 14 or 17.

Thereafter, during an accumulation period T7, electrons are transferredfrom the photodiode 111 to the intermediate accumulation node by timedivision by intermittently applying a pulse without continuously openingthe first and second NMOS transistors 1121 and 1122 by driving of thegate electrode 201.

During the accumulation period, the third NMOS transistor 1123 ismaintained in the OFF state through the separation gate electrode 202,and the intermediate accumulation node is maintained in the intermediatestate of step ST23 of FIG. 13(C) other than the above-describedintermittent transmission time.

The pulse is applied to the gate electrode 201 again during a readoperation, and the first and second NMOS transistors 1121 and 1122 areturned on, and thus electrons remaining in the photodiode 111 aretransferred to the intermediate accumulation node.

Ultimately, the pulse is applied to the separation gate electrode 202,the third NMOS transistor 1123 is turned on, and the accumulatedelectrons of the intermediate accumulation node are togethercollectively transferred to the FD 117, which is the amplifier input.

Like the seventh embodiment, the eighth embodiment can suppress a darkcurrent from being increased while implementing large-capacityaccumulation.

[Global Shutter Function]

Next, the global shutter function will be described.

The global shutter function is a function of removing focal planedistortion generated by the variation of a shutter timing within a pixelarray.

If a normal circuit configuration and sequence are adopted, the start ofthe read operation decides an accumulation end timing as shown in FIG.2.

Because the read is normally sequentially performed for each row, theaccumulation end timing also follows it. Consequently, normally, thereset operation serving as the accumulation start also varies for eachrow and is sequentially performed, and the accumulation period T1 istaken uniformly in all effective pixels.

This is a general method in a CMOS image sensor called a rollingshutter, and means that the shutter timing varies for each row. Forexample, distortion occurs in an image of a subject operating at highspeed.

On the other hand, the global shutter function is implemented bysimultaneously collectively starting electric charge accumulations ofall the effective pixels and further simultaneously collectivelyterminating the accumulations.

On the other hand, in this case, because accumulated data is read foreach row, it is necessary to separate the accumulation end timing andthe read timing, and it is necessary to store a signal for each pixelduring a period from the accumulation end to the read.

If the configuration of the transfer circuit shown in FIG. 5 is used andits intermediate retention mode is used, the above-described operationand good signal storage are possible.

That is, in the CMOS image sensor 100, the first and second NMOStransistors 1121 and 1122 are simultaneously turned off through the gateelectrode 201 with respect to all the effective pixels. Thereby, theaccumulation is terminated by stopping the transfer of photoelectricallyconverted electrons, and also already accumulated electrons are storedonce in the channel region of the second NMOS transistor 1122.

Thereafter, it is preferable to sequentially turn on the third NMOStransistor 1123 by applying a pulse to the gate electrode 202 with theread in units of rows and transfer stored electrons to the FD 117, whichis the input node of the amplifier circuit 118.

<9. Ninth Embodiment>

FIGS. 20(A) to 20(D) are diagrams showing a timing chart of theoperation of a ninth embodiment in which the global shutter function ismounted in the first embodiment.

FIG. 20(A) shows a signal potential of the reset line 150, FIG. 20(B)shows a signal potential of the transfer line 140, FIG. 20(C) shows asignal potential of the row selection line 160, and FIG. 20(D) shows asignal potential of the vertical signal line 170.

In the ninth embodiment, the pixel circuit is based on FIG. 4, and thepotential change is based on FIG. 7.

The reset line 150 has the high level during reset, so that the FD 117,which is the input node of the amplifier circuit 118, is connected to areset level (power supply voltage 3 V).

On the other hand, at a point in time when the level of the transferline 140 has risen from the low level to the high level and has furtherfallen from the high level to the low level, electrons accumulated inthe photodiode 111 are transferred to the FD 117 and drawn to the resetlevel.

At this point in time, a new electron accumulation period T8 starts. Apulse of the reset line 150 falls to the low level after waiting for apulse applied to the gate electrode 201 to fall to the low level.

Normally, this reset operation is sequentially performed for eachselected row, but the global reset is performed for all the effectivepixels at the same time.

That is, this process serves as a shutter opening operation of theglobal shutter.

At an accumulation end time when a predetermined accumulation time T8has elapsed, first, the level of the transfer line 140 rises from thelow level to the high level, so that electrons accumulated in thephotodiode 111 are transferred to the intermediate accumulation node. Atthis time, a potential state corresponds to step ST12 of FIG. 7(B).

Further, if the transfer line 140 returns from the high level to theintermediate potential, the potential state moves to the intermediateretention mode of step ST13 of FIG. 7(B) and the intermediateaccumulation node and the photodiode 111 are separated.

These are simultaneously performed for all the effective pixels, andserve as a shutter closing operation of the global shutter.

The read is sequentially performed for each row according to a rowaddress.

First, after a selection signal is applied to the row selection line 160and row selection is selectively performed, a pulse is applied to thereset line 150, the FD 117, which is an amplifier input section, isconnected to a reset level, and the reset level is sensed.

Next, the level of the transfer line 140 falls from the intermediatepotential to the low level, so that all electrons retained in theintermediate accumulation node are transferred to the FD 117, which isthe input node of the amplifier circuit 118.

Each pixel maintains the intermediate retention mode during a period ofH8 from the accumulation end to the read, but the intermediate retentionperiod H8 is different for each row.

That is, the above-described intermediate retention is performed duringa period until it has a turn to read a corresponding row after theshutter is closed at once.

<10. Tenth Embodiment>

FIGS. 21(A) to 21(E) are diagrams showing a timing chart of theoperation of a tenth embodiment in which the global shutter function ismounted in the third embodiment.

FIG. 21(A) shows a signal potential of the reset line 150, FIG. 21(B)shows a signal potential of the first transfer line 141, and FIG. 21(C)shows a signal potential of the second transfer line (separation line)142. FIG. 21(D) shows a signal potential of the row selection line 160,and FIG. 21(E) shows a signal potential of the vertical signal line 170.

In the tenth embodiment, a pixel circuit is the same as that of FIG. 12,and a potential change of the transfer is based on FIGS. 13(A) to 13(D).

The reset line 150 has the high level during reset, so that the FD 117,which is the input node of the amplifier circuit 118, is connected to areset level (power supply voltage 3 V).

On the other hand, the level of the first transfer line 141 rises fromthe low level to the high level, so that extra electrons accumulated inthe photodiode 111 are transferred to the channel portion of the secondNMOS transistor 1122 via the first NMOS transistor 1121.

Further, the level of the first transfer line 141 falls from the highlevel to the low level and the second transfer line 142 as theseparation line has the high level at substantially the same time, sothat the third NMOS transistor 1123 for separation is conductive.Ultimately, if the level of the second transfer line 142 falls to thelow level, the accumulated electrons are completely transferred to thediffusion layer 205 and drawn to the reset level.

At this point in time, a new electron accumulation period T9 starts.Normally, this reset operation is sequentially performed for eachselected row, but the global reset is performed for all effective pixelsat the same time.

That is, this process serves as a shutter opening operation of theglobal shutter.

At an accumulation end time when a predetermined accumulation time T9has elapsed, first, the level of the first transfer line 141 rises fromthe low level to the high level, so that electrons accumulated in thephotodiode 111 are transferred to the intermediate accumulation node. Atthis time, a potential state corresponds to step ST22 of FIG. 13(B).

Further, if the first transfer line 141 returns from the high level tothe low level, the potential state moves to the intermediate retentionmode of step ST23 of FIG. 13(B) and the intermediate accumulation nodeand the photodiode are separated.

These are simultaneously performed for all the effective pixels, andserve as a shutter closing operation of the global shutter.

The read is sequentially performed for each row according to a rowaddress.

First, after a selection signal is applied to the row selection line 160and row selection is selectively performed, a pulse is applied to thereset line 150, the FD 117, which is the input node of the amplifiercircuit 118, is connected to a reset level, and the reset level issensed.

Next, the second transfer line 142 as the separation line is driven tothe high level, and the third NMOS transistor 1123, which is theseparation transistor, is conductive. Thereby, electrons accumulated inthe channel portion of the second NMOS transistor 1122 are transferredto the FD 117, which is the input node of the amplifier 118.

At a point in time when the second transfer line 142 as the separationline has fallen again to the low level, the complete transfer ofaccumulated electrons to the diffusion layer 205 is completed.

Each pixel maintains the intermediate retention mode during a period ofH9 from the accumulation end to the read, but the intermediate retentionperiod H9 is different for each row.

That is, the above-described intermediate retention is performed duringa period until it has a turn to read a corresponding row after theshutter is closed at once.

The global shutter function can also be performed in combination withthe above-described large-capacity accumulation operation.

For example, when the accumulation of a global shutter sequence shown inFIGS. 20(A) to 20(D) is started, transfer lines 140 of all effectivepixels are collectively changed to the high level. It is preferable toterminate the accumulation by collectively dropping them to theintermediate potential after maintaining their state during theaccumulation period T8 and move to the intermediate retention state.

In this case, the potential state of step ST12 of FIG. 7(B) ismaintained in each pixel during the accumulation period, and electronsare accumulated within the channel of the second NMOS transistor 1122,not in the photodiode 111, so that large-capacity accumulation ispossible.

Likewise, a combination with a large-capacity accumulation operation bya time division transfer is also possible, and the large-capacityaccumulation operation and the global shutter function can be combinedand used along with a basic function of the first embodiment and a basicconfiguration of the third embodiment.

[Wide Dynamic Range Function]

Next, the wide dynamic range function will be described.

This function is a function of simultaneously storing a signal of ashort accumulation time and a signal of a long accumulation time andpicking up an image in an exposure time simultaneously appropriate forthe two signals using the signal of the short accumulation time forsensing a subject of high luminance and the signal of the longaccumulation time for sensing a subject of low luminance.

If the configuration of the first or third embodiment of the presentinvention and the intermediate retention mode are applied, it ispossible to accumulate a separate signal in the photodiode while storinga signal accumulated for a long time in the channel region of thetransistor in the intermediate retention mode.

During a read operation, the signal of the long-time accumulation sidestored in the intermediate retention mode is first transferred to theamplifier input, and then the signal of the short-time accumulation sidestored in the photodiode is transferred.

<11. Eleventh Embodiment>

An example of the wide dynamic range operation using the configurationof the first embodiment will be described using potential changes ofFIGS. 22(A) to 22(D) and FIGS. 23(A) to 23(C).

FIGS. 22(A) to 22(D) are first potential change diagrams illustrating anexample of the wide dynamic range operation according to an eleventhembodiment using the configuration of the first embodiment.

FIGS. 23(A) to 23(C) are second potential change diagrams illustratingan example of the wide dynamic range operation according to the eleventhembodiment using the configuration of the first embodiment.

In the eleventh embodiment, a pixel circuit is the same as shown in FIG.4, details and a cross-sectional configuration of a transfer circuit arethe same as shown in FIGS. 5 and 6.

[Step ST31]

In step ST31 of FIG. 22(A) like step ST11 of FIG. 7(A), first electronaccumulation is performed in the diffusion node 204 of the photodiode111.

In the channel regions of the first NMOS transistor 1121 and the secondNMOS transistor 1122, potentials are respectively modulated in ranges ofR11 and R12 according to a potential commonly applied to the shared gateelectrode 201 of the two, for example, −1.5 to 3 V.

On the other hand, the gate electrode 202 of the third NMOS transistor1123, which is the separation transistor, is connected to the groundGND, and the potential of the channel is adjusted to about 0.6 V.

[Step ST32]

If the first NMOS transistor 1121 and the second NMOS transistor 1122are turned on in step ST32 of FIG. 22(B), the electrons move.

Electrons accumulated in the diffusion node 204 of the photodiode 111all move to the channel region of the second NMOS transistor 1122 viathe first NMOS transistor 1121.

That is, the electrons move to the channel region of the second NMOStransistor 1122 having a deep depletion state and are accumulated in ananalog state.

[Step ST33]

The gate electrode 201 is driven to turn off the first NMOS transistor1121 and the second NMOS transistor 1122 in step ST33 of FIG. 22(C), sothat the potential of the channel region is modulated in the negativepotential direction.

Thereby, the photodiode 111 is separated from the channel of the secondNMOS transistor 1122, and first accumulation is completed.

In the step in which the gate electrodes of the first NMOS transistor1121 and the second NMOS transistor 1122 have reached an appropriateintermediate voltage, a state is reached in which the accumulatedelectrons have been separated from both the photodiode 111 and the FD117, which is the input node of the amplifier circuit 118.

[Step ST34]

Because light is continuously incident on the photodiode 111 andphotoelectrically converted if the intermediate retention state of stepST33 is maintained in step ST34 of FIG. 22(D), new electrons areaccumulated in its diffusion node 204.

[Step ST35]

If the first NMOS transistor 1121 and the second NMOS transistor 1122are completely turned off in step ST35 of FIG. 23(A), the electrons moveas follows.

That is, all first accumulated electrons retained in the channel of thesecond NMOS transistor 1122 move to the FD 117, which is the input nodeof the amplifier circuit 118.

Thereby, the amplifier circuit 118 having the amplifier transistor 114drives the vertical signal line 170, and a first accumulated signal isread.

[Step ST36]

If the first NMOS transistor 1121 and the second NMOS transistor 1122are turned on again in step ST36 of FIG. 23(B), the electrons move asfollows.

Second accumulated electrons accumulated in the diffusion node 204 ofthe photodiode 111 all move to the channel region of the second NMOStransistor via the first NMOS transistor 1121. At this time, the FD 117,which is the input node of the amplifier circuit 118, is reset to 3 V.

[Step ST37]

If the first NMOS transistor 1121 and the second NMOS transistor 1122are completely turned off again in step ST37 of FIG. 23(C), theelectrons move as follows.

All of the second accumulated electrons retained in the channel of thesecond NMOS transistor 1122 move to the FD 117, which is the input nodeof the amplifier circuit.

Thereby, the amplifier circuit 118 having the amplifier transistor 114drives the vertical signal line 170, and a second accumulated signal isread.

By adjusting an effective timing of the above-described operationsequence, the first electron accumulation is performed for a long timeand the second electron accumulation is performed for a short time.

If the first electron accumulation is not saturated, its value is usedin accumulated data of a pixel. On the other hand, if the first electronaccumulation is saturated, a value of the second electron accumulationis used in the accumulated data of the pixel. If the second accumulationtime is 1/K of the first accumulation time, second accumulated data ishandled to be K times during image synthesis.

The long-time accumulation and the short-time accumulation arecontinuously performed without the read in the middle. The read issequentially continuously performed twice for each row.

Therefore, frame synthesis is possible if a user of an imager accordingto the embodiment of the present invention prepares only two linebuffers without having to prepare two frame buffers corresponding todifferent accumulation times.

When the read time is doubled, a frame rate becomes ½ but all thedoubled time taken for one frame can be used in accumulation.

FIGS. 24(A) to 24(D) are diagrams showing a timing chart of theabove-described wide dynamic range correspondence operation.

FIG. 24(A) shows a signal potential of the reset line 150, FIG. 24(B)shows a signal potential of the transfer line 140, FIG. 24(C) shows asignal potential of the row selection line 160, and FIG. 24(D) shows asignal potential of the vertical signal line 170.

First, the reset line 150 is set to the high level during reset, so thatthe FD 117, which is the input node of the amplifier circuit 118, isconnected to a reset level (power supply voltage 3 V).

On the other hand, at a point in time when the level of the transferline 140 has risen from the low level to the high level and has furtherfallen from the high level to the low level, electrons accumulated inthe photodiode 111 are transferred to the FD 117 and drawn to the resetlevel.

At this point in time, an accumulation period T10L of the first electronaccumulation starts. A pulse of the reset line 150 falls to the lowlevel after waiting for the level of the transfer line 140 to fall tothe low level.

When a predetermined accumulation time has elapsed, the level of atransfer line rises from the low level to the high level, andaccumulated electrons are transferred to the intermediate node formed inthe channel portion of the second NMOS transistor 1122 as shown in stepST32 of FIG. 22(B).

Further, if the level of the transfer line 140 falls from the high levelto the intermediate potential, the photodiode 11 and the intermediatenode are disconnected as shown in step ST33 of FIG. 22(C), and theaccumulation period T10L of the long-time side of the first electronaccumulation ends.

Simultaneously, a second accumulation period T10S starts.

After a selection signal is applied to the row selection line 160 androw selection is performed, the read is performed as follows.

First, the FD 117, which is the input node of the amplifier circuit 118,is reset by a pulse application of the reset line 150, and the resetlevel is sensed.

Next, if the level of the transfer line 140 falls to the low level fromthe intermediate node, the first accumulated electrons are transferredto the FD 117, which is the input node of the amplifier circuit 118, asshown in step ST35 of FIG. 23(A), and their sensing is performed.

The FD 117, which is the input node of the amplifier circuit 118, isreset again by a pulse application of the reset line 150, and the resetlevel is sensed.

Next, if the pulse is applied to the transfer line 140, the secondaccumulated signal is transferred to the FD 117, which is the input nodeof the amplifier circuit 118, through steps ST36 and ST37 of FIGS. 23(B)and 23(C), and its sensing is performed.

The accumulation period T10S also ends by reading the second accumulatedsignal.

<12. Twelfth Embodiment>

The above-described wide dynamic range function is also performed withthe same concept for the configuration of the third embodiment.

FIGS. 25(A) to 25(E) are diagrams showing a timing chart of the widedynamic range operation according to a twelfth embodiment using theconfiguration of the third embodiment.

In the twelfth embodiment, a configuration of a pixel configuration isthe same as that of FIG. 12.

The reset line 150 has the high level during reset, so that the FD 117,which is the input node of the amplifier circuit 118, is connected to areset level (power supply voltage 3 V).

On the other hand, the level of the first transfer line 141 rises fromthe low level to the high level, so that extra electrons accumulated inthe photodiode 111 are transferred to the channel portion of the secondNMOS transistor 1122 via the first NMOS transistor 1121.

Further, the level of the first transfer line 141 falls from the highlevel to the low level and the second transfer line 142 as theseparation line has the high level at substantially the same time, sothat the third NMOS transistor 1123, which is the separation transistor,is conductive. Ultimately, if the level of the second transfer line 142falls to the low level, accumulated electrons are completely transferredto the FD 117 and drawn to the reset level.

At this point in time, an accumulation period T11L of the first electronaccumulation starts. A pulse of the reset line 150 falls to the lowlevel after waiting for the level of the second transfer line 142 tofall to the low level.

If a predetermined accumulation time has elapsed, the level of the firsttransfer line 141 rises from the low level to the high level, so thatelectrons accumulated in the photodiode 111 are transferred to theintermediate accumulation node.

Further, if the first transfer line 141 returns from the high level tothe low level, the potential state moves to the intermediate retentionmode of step ST23 of FIG. 13(B) and the intermediate accumulation nodeand the photodiode are separated.

Thereby, the accumulation period T11L of the long-time side of the firstelectron accumulation ends. Simultaneously, a second accumulation periodT11S starts.

After a selection signal is applied to the row selection line 160 androw selection is performed, the read is performed as follows.

First, a pulse is applied to the reset line 150, the FD 117, which isthe input node of the amplifier circuit 118, is connected to the resetlevel, and the reset level is sensed.

Next, the second transfer line 142 as the separation line is driven tothe high level, and the third NMOS transistor 1123 as the separationtransistor is conductive. Thereby, the first accumulated electronsaccumulated in the channel portion of the second NMOS transistor 1122are transferred to the FD 117, which is the input node of the amplifiercircuit 118. At a point in time when the second transfer line 142 as theseparation line has fallen again to the low level, the complete transferof the first accumulated electrons to the FD 117 is completed and thefirst accumulated signal is sensed.

The FD 117, which is the input node of the amplifier circuit 118, isreset again by a pulse application of the reset line 150, and the resetlevel is sensed.

Next, the first transfer line 141 is driven from the low level to thehigh level, so that the second accumulated electrons accumulated in thephotodiode 111 are transferred to the channel portion of the second NMOStransistor 1122 via the first NMOS transistor 1121.

Further, if the level of the first transfer line 141 falls from the highlevel to the low level, the second transfer line 142 as the separationline has the high level at substantially the same time, and ultimatelythe level of the second transfer line 142 falls to the low level, thesecond accumulated electrons are completely transferred to the FD 117.

Thereby, the second accumulation period T11S also ends, and the secondaccumulated signal is subsequently sensed.

If the first electron accumulation is not saturated, its value is usedin accumulated data of a pixel. On the other hand, if the first electronaccumulation is saturated, a value of the second electron accumulationis used in the accumulated data of the pixel. If the second accumulationtime is 1/K of the first accumulation time, second accumulated data ishandled to be K times during image synthesis.

<13. Thirteenth Embodiment>

Next, the twelfth embodiment in which the structure of a transfercircuit within a pixel is changed will be described.

FIG. 26 is a diagram showing a pixel circuit of a CMOS image sensoraccording to a thirteenth embodiment of the present invention.

A pixel circuit 110E according to the thirteenth embodiment has aconfiguration in which the third NMOS transistor 1123 as the separationtransistor of the transfer transistor 112 of the pixel circuit 110Aaccording to the first embodiment is omitted.

That is, in the pixel circuit 110E according to the thirteenthembodiment, a transfer transistor 112E is formed of the first MOStransistor 1121 of the high threshold voltage HVth, and the second MOStransistor 1122 of the low threshold voltage LVth integrated andconnected in series.

FIG. 27 is a diagram showing an equivalent circuit of the transfercircuit including the transfer transistor of the pixel circuit 110Eaccording to the thirteenth embodiment.

Electrons generated by photoelectric conversion in the photodiode 111are completely transferred to the FD 117, which is the input node of theamplifier circuit 118, via the first and second NMOS transistors 1121and 1122 integrated and connected in series to form the transfertransistor 112E.

In the integrated first and second NMOS transistors 1121 and 1122,channels are directly connected to each other, not via an n-typediffusion layer or the like.

A driving signal is collectively applied to the gate electrodes 201 ofthe first and second NMOS transistors 1121 and 1122 at the same time.

The first NMOS transistor 1121 has the high threshold voltage HVth, andthe second NMOS transistor 1122 has the low threshold voltage LVth.

The FD 117, which is the input node, has the parasitic capacitance 203,and its potential variation amount ΔVf is {ΔVf=Q/Cf} if an accumulatedelectric charge amount is Q and a parasitic capacitance value is Cf.

During a read operation, this displacement drives the vertical signalline 170 at a fixed gain via the amplifier circuit 118.

As the third NMOS transistor as the separation transistor is omittedfrom the first embodiment, an area occupied by a pixel is reduced by theomission.

On the other hand, the FD 117, which is the input node of the amplifiercircuit 118 in a floating state, is easily affected by a state change ofthe second NMOS transistor 1122 adjacent thereto.

For example, when the first and second NMOS transistors 1121 and 1122are turned on through the gate electrodes 201, the potential of the FD117 is changed by coupling thereof. As a result, there is the effectthat some electrons to be accumulated in the channel portion of thesecond NMOS transistor 1122 are leaked to the FD 117, which is the inputnode of the amplifier circuit 118, or the like.

The FD 117, which is the input node of the amplifier circuit 118,includes a diffusion layer into which a large amount of impurities isintroduced, a contact portion of a wiring, or the like, and has lowcrystallinity as compared to the channel portion of the MOS transistor.

Therefore, individually leaked electrons are easily lost byrecombination or the like during the accumulation period, andparticularly have significantly negative influence on the accumulationfunction of the third embodiment or the global shutter function of theninth or tenth embodiment.

However, if electric charge accumulation capability is sufficientlylarge in the channel portion of the second NMOS transistor 1122, it ispossible in the thirteenth embodiment also to reduce or remove a marginlimitation of a complete transfer in a principal similar to that of thefirst embodiment.

FIG. 28 is a diagram showing a cross-sectional structure example of thetransfer circuit of FIG. 27.

In the photodiode 111, a HAD structure in which the vicinity of asilicon surface in contact with an oxide film has a p-type is adopted.

Here, the photoelectrically converted electrons are initiallyaccumulated in the n-type diffusion node 204.

If a signal that turns on the first NMOS transistor 1121 is applied tothe gate electrode 201, many electrons are transferred to the channelregion of the second NMOS transistor 1122 via the first NMOS transistor1121, and accumulated in the channel region.

For example, an impurity profile of a channel portion is adjusted, sothat a threshold of the first NMOS transistor 1121 is set to be high anda threshold of the second NMOS transistor 1122 is set to be low.Thereby, the channel portion of the second NMOS transistor 1122 forms anelectron accumulation well, and the channel portion of the first NMOStransistor 1121 forms a potential wall of backflow prevention.

The diffusion layer 205 is connected to an input of the amplifiercircuit 118, which is not shown in the cross-sectional view.

Here, the first NMOS transistor 1121 and the second NMOS transistor 1122are considered two individual transistors. However, they can beconsidered a single NMOS transistor having a gradient in the impurityprofile of the channel portion if the gate electrodes are alsointegrally formed as shown in the figure.

In any case, it is functionally the same as two individual transistorsconnected in series, and is included in an application range of thepresent invention.

FIGS. 29(A) to 29(D) are diagrams showing potential changes accompanyinga read transfer operation using the transfer circuit of the pixelcircuit according to the thirteenth embodiment.

[Step T41]

In step ST41 of FIG. 29(A), the diffusion node 204 of the photodiode 111is designed so that a potential bottom is about 2.5 V during fulldepletion in positive electric charge by a fixed number of donors. Here,it is filled with photoelectrically converted electrons up to asaturation state (about 0 V).

On the other hand, in the channel regions of the first NMOS transistor1121 and the second NMOS transistor 1122, potentials are respectivelymodulated in ranges of R17 and R18 according to a potential commonlyapplied to the two gate electrodes, for example, 1.5 V to 3 V.

The diffusion layer 205 (the FD 117), which is the input node of theamplifier circuit 118, is reset to have a floating state of 3 V.

[Step ST42]

If the first NMOS transistor 1121 and the second NMOS transistor 1122are turned on in step ST42 of FIG. 29(B), a potential moves as follows.

Electrons accumulated in the diffusion node 204 of the photodiode 111all move to the channel region of the second NMOS transistor 1122 viathe first NMOS transistor 1121.

In this case, the potential of the diffusion layer 205 (the FD 117),which is the input node of the amplifier circuit 118, rises by coupling.Some electrons further flow into the diffusion layer 205 (the FD 117)via the channel portion of the second NMOS transistor 1122.

That is, most electrons, which are a read signal, are accumulated in thechannel region of the second NMOS transistor 1122 having a deepdepletion state, and some electrons are accumulated in the diffusionlayer 205 (the FD 117), which is the input node of the amplifier circuit118.

[Step ST43]

The gate electrodes 201 are driven to turn off the first NMOS transistor1121 and the second NMOS transistor 1122 in step ST43 of FIG. 29(C), sothat the potential of the channel region is modulated in the negativedirection.

Here, the channel of the first NMOS transistor 1121 forms a potentialbarrier, and prevents accumulated electrons from flowing back to thediffusion node 204 of the photodiode 111. A height of the barriercorresponds to a difference between thresholds of the two transistors,for example, 1.5 V.

As the potential of the channel region of the second NMOS transistor1122 rises (the potential decreases), electrons accumulated therein moveto the diffusion layer 205 (the FD 117), which is the input node of theamplifier circuit 118.

[Step ST44]

In an OFF state of the first and second NMOS transistors 1121 and 1122in step ST44 of FIG. 29(D), a state is reached in which all electronsaccumulated in the photodiode in step S41 have moved to the diffusionlayer 205, which is the input node of the amplifier circuit 118.Thereby, the amplifier drives the vertical signal line, and anaccumulated signal is read.

If the above-described step-by-step transfer is used, it is unnecessaryto secure a potential difference between the diffusion node 204 of thephotodiode 111 having the full depletion state and the diffusion layer205, which is the input node of the amplifier circuit 118.

That is, in the thirteenth embodiment, a complete transfer isimplemented even in a state in which the potential of the diffusionlayer 205 (the FD 117) filled with electrons is shallower than that ofthe diffusion node 204.

To completely remove the above-described potential limitation during thetransfer, the accumulation capacity of the channel portion of the secondNMOS transistor 1122 is sufficiently increased in step ST42. Thereby, itis necessary to move all electrons to the right side from the secondNMOS transistor 1122, regardless of the magnitude of the parasiticcapacitance of the diffusion layer 205, which is the input node of theamplifier circuit 118.

If the saturated accumulated electric charge amount of the photodiode isQs, a channel capacity of the second NMOS transistor 1122 (a capacity ofan inversion layer) is Cinv, and a threshold difference between thefirst NMOS transistor 1121 and the second NMOS transistor 1122 is ΔVth,the following condition is given.|Cinv*ΔVth|>|Qs|  [Expression 3]

Actually, there is an advantageous effect in that a transfer marginsufficiently significantly extends if a state is reached in which halfor more of electrons generated by photoelectric conversion of thephotodiode 111 have been accumulated in the channel portion of thesecond NMOS transistor 1122 in step ST42.

<14. Fourteenth Embodiment>

FIG. 30 is a diagram showing a pixel circuit of a CMOS imager accordingto a fourteenth embodiment of the present invention.

A difference between a pixel circuit 110F according to the fourteenthembodiment and the pixel circuit 110E according to the thirteenthembodiment is as follows.

In the pixel circuit 110F according to the fourteenth embodiment, aplurality of pixels, for example, two pixels PXL110 a and PXL110 b,respectively having a unique photodiode 111 and a unique transfercircuit 112, share FD 117 and an amplifier transistor 114 forming anamplifier circuit.

In the pixel circuit 110F, the plurality of pixels PXL110 a and PXL110 balso share a reset transistor 113 and a row selection transistor 115.

In transfer transistors 112 a and 112 b of the respective pixels PXL110a and PXL110 b, shared gate electrodes of first and second NMOStransistors are respectively connected to different transfer lines 140 aand 140 b.

In the pixel circuit 110F, electrons accumulated in respectivephotodiodes 111 a and 111 b are transferred to the FD 117 (the inputnode of the amplifier circuit) at individual timings according to therespectively independent transfer lines 140 a and 140 b.

The sharing of the amplifier circuit can reduce an effective size of thepixel, but parasitic capacitance of the FD 117 also increases when thenumber of sharing pixels increases.

Therefore, it is preferable that the number of sharing pixels be equalto or greater than 2 and equal to or less than 16.

The embodiments using the photodiode in a photoelectric conversionelement of a semiconductor imager have been described above.

On the other hand, a MOS capacitor may be used in the photoelectricconversion element. It is possible to obtain the same effect even whenthe MOS capacitor is used in place of the photodiode in the first tofourteenth embodiments.

<15. Fifteenth Embodiment>

FIG. 31 is a diagram showing a configuration example of a transfercircuit according to a fifteenth embodiment in which the photodiode isreplaced with a MOS capacitor with respect to FIG. 6, which is across-sectional structure example corresponding to the first embodiment.

In FIG. 31, reference numeral 210 denotes a photoelectric conversionelement using a MOS capacitor.

For example, a fixed voltage of 2 V is applied to an electrode 211, anda MOS capacitor 210 has a deep depletion state.

If electrons enter the depletion layer, electron/hole pairs aregenerated. Holes are attracted to an electric field and go to a p-wellside. On the other hand, the electrons are accumulated in the vicinityof an oxide film of the MOS capacitor 210 as an inversion layer.

If the gate electrode 201 has the high level, the accumulated electronsare completely transferred to the channel region of the second NMOStransistor 1122 via the first MOS transistor 1121, and accumulated inthe channel region.

Further, if the level of the gate electrode 201 falls to the low level,the accumulated electrons are transferred to the diffusion layer 205(the FD 117), which is the input node of the amplifier circuit, thevertical signal line 170 is driven, and the read is performed.

<16. Sixteenth Embodiment>

FIG. 32 is a diagram showing a cross-sectional structure example of atransfer circuit according to a sixteenth embodiment having across-sectional structure different from the transfer circuit of thefirst embodiment.

A main difference between the transfer circuit according to thesixteenth embodiment of FIG. 32 and the transfer circuit according tothe first embodiment of FIG. 6 is an integrated gate structure of theintegrated first to third NMOS transistors 1121, 1122, and 1123.

In the sixteenth embodiment, the first NMOS transistor 1121 and thesecond NMOS transistor 1122 are formed using different gate electrodes201-1 and 201-2.

The gate electrodes 201-1 and 201-2 are formed of different conductivelayers or polysilicon layers, and short-circuited within a pixel (notshown) to form an integrated electrode 201.

In this structure, it is possible to adjust a substrate impurity profileof the second NMOS transistor 1122 in self alignment. Alternatively, itis also possible to adjust a threshold by changing work functions ofdifferent gate electrode layers.

The solid-state image pickup device according to the first to sixteenthembodiments described above may be applied as an image pickup device ofa digital camera or a video camera.

<17. Seventeenth Embodiment>

FIG. 33 is a diagram showing an example of a configuration of a camerasystem to which a solid-state image pickup device is applied accordingto an embodiment of the present invention.

As shown in FIG. 33, a camera system 300 has an image pickup device 310to which the CMOS image sensors (solid-state image pickup devices) 100is applicable.

The camera system 300 has an optical system, which guides incident lightto a pixel region of the image pickup device 310 (or which forms animage of a subject), for example, a lens 320, which forms an image ofthe incident light (an optical image) on an image pickup surface.

Further, the camera system 300 has a driving circuit (DRV) 330, whichdrives the image pickup device 310, and a signal processing circuit(PRC) 340, which processes an output signal of the image pickup device310.

The driving circuit 330 has a timing generator (not shown), whichgenerates various timing signals including a start pulse or a clockpulse to drive a circuit within the image pickup device 310, and drivesthe image pickup device 310 by a predetermined timing signal.

The signal processing circuit 340 performs predetermined signalprocessing for the output signal of the image pickup device 310.

An image signal processed by the signal processing circuit 340 isrecorded, for example, on a recording medium such as a memory. Imageinformation recorded on the recording medium is hard-copied by a printeror the like. The image signal processed by the signal processing circuit340 is displayed on a monitor including a liquid crystal display or thelike as a moving image.

In an image pickup apparatus of a digital still camera or the like asdescribed above, a high-precision camera can be implemented at low powerconsumption by mounting the above-described image pickup device 100 asthe image pickup device 310.

REFERENCE SIGNS LIST

-   -   100: CMOS image sensor    -   110: Pixel array section    -   110A to 110F: Pixel circuit    -   111: Photodiode    -   112: Transfer transistor    -   1121: First MOS transistor    -   1122: Second MOS transistor    -   1123: Third MOS transistor    -   113: Reset transistor    -   114: Amplifier transistor    -   115: Row selection transistor    -   116: Accumulation node    -   117: FD    -   118: Amplifier circuit    -   120: Row selection circuit    -   130: Column read circuit (AFE)    -   300: Camera system

The invention claimed is:
 1. A pixel circuit comprising: a photoelectricconversion element; an amplifier circuit; and a transfer transistor fortransferring electric charge generated by the photoelectric conversionelement to an input node of the amplifier circuit, wherein the transfertransistor has first, second, and third field effect transistorsintegrated and connected in series from the photoelectric conversionelement to a side of the amplifier circuit, the first and second fieldeffect transistors have gate electrodes to be simultaneouslycollectively driven, and a threshold voltage of the first field effecttransistor is set to be higher than that of the second field effecttransistor, as the gate electrodes are driven step by step, the electriccharge generated by the photoelectric conversion element and transferredvia the first field effect transistor is accumulated in a channel regionof the second field effect transistor, the electric charge accumulatedin the channel region is transferred to an input of the amplifiercircuit via the third field effect transistor, the amplifier circuitdrives a signal line, so that the accumulated electric charge is read,and a gate electrode of the third field effect transistor has a fixedpotential.
 2. The pixel circuit according to claim 1, wherein the thirdfield effect transistor transfers the electric charge from the channelregion of the second field effect transistor to the input of theamplifier circuit if a pulse is applied to a gate electrode.
 3. Thepixel circuit according to claim 1, wherein the transfer transistor hasa function of retaining the electric charge accumulated in the channelregion of the second field effect transistor during a predeterminedperiod as the gate electrodes of the first and second field effecttransistors are maintained at an intermediate voltage.
 4. The pixelcircuit according to claim 1, wherein the transfer transistor has afunction of retaining the electric charge accumulated in the channelregion of the second field effect transistor by maintaining the gateelectrodes of the first and second field effect transistors and the gateelectrode of the third field effect transistor together at apredetermined level.
 5. The pixel circuit according to claim 1, whereinthe transfer transistor has an accumulation function of accumulating theelectric charge generated by the photoelectric conversion element bymaintaining the first field effect transistor in an ON state anddirectly transferring the electric charge to the channel region of thesecond field effect transistor, and a read function of transferring theaccumulated electric charge to the amplifier circuit via the third fieldeffect transistor.
 6. The pixel circuit according to claim 1, whereinthe transfer transistor accumulates a first accumulated signal generatedby the photoelectric conversion element by transferring the firstaccumulated signal to the channel region of the second field effecttransistor via the first field effect transistor, the first field effecttransistor is turned off while second accumulation is started by thephotoelectric conversion element and a second accumulated signal isgenerated in a state in which the electric charge has been accumulatedin the channel region of the second field effect transistor, thetransfer transistor performs first read, which transfers the firstaccumulated signal to the input of the amplifier circuit via the thirdfield effect transistor, and the transfer transistor performs secondread, which transfers the second accumulated signal to the amplifiercircuit via the first, second, and third field effect transistors. 7.The pixel circuit according to claim 1, wherein the photoelectricconversion element and the transfer transistor including the first,second, and third field effect transistors are arranged in each of aplurality of pixels, and the amplifier circuit is shared between theplurality of pixels.
 8. A solid-state image pickup device comprising: apixel section where a plurality of pixel circuits are arranged; and apixel driving section for reading a pixel signal by driving a pixelcircuit of the pixel section, wherein each pixel circuit has aphotoelectric conversion element; an amplifier circuit; and a transfertransistor for transferring electric charge generated by thephotoelectric conversion element to an input node of the amplifiercircuit, wherein the transfer transistor has first, second, and thirdfield effect transistors integrated and connected in series from thephotoelectric conversion element to a side of the amplifier circuit, thefirst and second field effect transistors have gate electrodes to besimultaneously collectively driven, and a threshold voltage of the firstfield effect transistor is set to be higher than that of the secondfield effect transistor, as the gate electrodes are driven by the pixeldriving section step by step, the electric charge generated by thephotoelectric conversion element and transferred via the first fieldeffect transistor is accumulated in a channel region of the second fieldeffect transistor, the electric charge accumulated in the channel regionis transferred to an input of the amplifier circuit via the third fieldeffect transistor, the amplifier circuit drives a signal line, so thatthe accumulated electric charge is read, and a gate electrode of thethird field effect transistor has a fixed potential.
 9. The solid-stateimage pickup device according to claim 8, wherein the transfertransistor accumulates the electric charge generated by thephotoelectric conversion element and transferred via the first fieldeffect transistor in the channel region of the second field effecttransistor as the gate electrodes are driven by the pixel drivingsection step by step, the first field effect transistor issimultaneously turned off for all effective pixels in a state in whichthe electric charge has been accumulated in the channel region of thesecond field effect transistor, the electric charge is then sequentiallytransferred to the input of the amplifier circuit via the third fieldeffect transistor for each row, and the amplifier circuit drives asignal line, so that the accumulated electric charge is read.
 10. Thesolid-state image pickup device according to claim 8, wherein thetransfer transistor accumulates a first accumulated signal generated bythe photoelectric conversion element by transferring the firstaccumulated signal to the channel region of the second field effecttransistor via the first field effect transistor, the first field effecttransistor is turned off while second accumulation is started by thephotoelectric conversion element and a second accumulated signal isgenerated in a state in which the electric charge has been accumulatedin the channel region of the second field effect transistor, thetransfer transistor performs first read, which transfers the firstaccumulated signal to the input of the amplifier circuit via the thirdfield effect transistor, and the transfer transistor performs secondread, which transfers the second accumulated signal to the amplifiercircuit via the first, second, and third field effect transistors. 11.The solid-state image pickup device according to claim 8, wherein thephotoelectric conversion element and the transfer transistor includingthe first and second field effect transistors are arranged in each of aplurality of pixels, and the amplifier circuit is shared between theplurality of pixels.